The United States was aware of the potential of RADAR for a decade before WWII began.

• In the autumn of 1922, Albert H. Taylor and Leo C. Young at the U.S. Naval Aircraft Radio Laboratory , suggested that radio waves could be used for ship detection in a harbor defense, but their suggestion was not taken up.
• In 1930, Lawrence A. Hyland and Albert A. Taylor at the U.S. Naval Research Laboratory (NRL) in Washington, D.C., used radio equipment to detect a passing aircraft, five years before Watts presented this ability to the British military.
• 1934, Americans Page, Taylor, and Young are credited with building and demonstrating the world’s first true radar.
• Scientists for the United States coined the term RADAR an acronym for Radio Detection And Ranging".

By the mid 1930's the investment and innovations in RADAR were coming out of Britain and Germany. Why hadn’t the US military pursued RADAR harder and why wasn’t the largest economy in the world(the United States) spending the most money on this important technology?

The US army prior to WWII was not an institution which had much financial support to pursue innovative high reward projects with long research cycles like radar. The United States had the largest economy in the world for decades by the time WWII began, but the United States maintained a small poorly funded military commensurate with a historic policy of isolation and neutrality in foreign wars. In 150 years the only "foreign war"** which the United States ever entered was WWI. The policy of neutrality and isolationism which had come down from George Washington in his farewell address was one which had significant popular and political support right up until Pearl Harbor.

** By "foreign war", I don't mean a war with a foreign power. I mean a war like WWI or WWII which began as a war between two foreign powers and the US entered due to an alliance or perceived interest.

Biennial Report, By the Chief of Staff of the United States Army To the Secretary of War 1 July 1939 to June of 1945

Source

"When General Marshal took office (Sept 1939), the 174,000-man U.S. Army ranked nineteenth in size in the world, behind Portugal and only slightly ahead of Bulgaria.

US Army Strength By Year

June of 1939...............................187,893
August of 1941.........................1,588,032
May of 1945 (peak strength)....8,291,336
document provided by James Tobias, U.S Army Center of Military History at Fort McNair in Washington, D.C. by email)

While the US economy was significantly larger than the British economy in the 1930's, the British military was approximately 5 times larger than the US military when WWII began (Sept 1939).

Why didn’t the United States adopt Radar sooner? Money and general national lack of commitment/interest in defense. The US populous and political leadership(outside the executive) was barely interested in preparing for the possibility of war.

On the eve of WWII, Congress nearly allowed 80% of the US Army to return to civilian life. The military build up being pursued by Roosevelt called for a 1 year enlistment and congress (by a single vote) just narrowly passed an extension. 203 - 202, August 1941. "the draft extension bill."

Even though Roosevelt believed war was inevitable. Roosevelt ran for office in 1940 promising to keep us out of WWII, because to do otherwise would have been politically unsupportable.

Roosevelt, acutely aware of strong isolationist and non-interventionism sentiment, promised there would be no involvement in foreign wars if he were re-elected.

The US was a committed isolationist country and the only thing that changed that was Pearl Harbor. Research on weapon systems with long development cycles just were not prioritized.

What's happening with other weapon systems?

• torpedo - The United States Navy didn’t have a working torpedo until the second half of the war(mark 14 torpedo Troubles,
  more on the mark 14).

• Rifles - The massive military buildup which the United States began in late 1939, first objective was to provide our troops with riffles, machine guns and tanks in Lew of the broomsticks for rifles and stove pipes for cannons they trained with.

• Training - You ever hear of the Air Mail Scandal of 1934? The United States Army Air Corps wasn't even competent enough in 1934 to fly the mail without killing 13 of it's pilots.

• Fighter Aircraft - Most pursuit fighters (In the United States Army Air Corps) before 1935 were (still biplanes) of the Curtiss P-1 Hawk (1926–1930) and Boeing P-12 (1929–1935) families

• Tanks - From the end of World War I to 1935, only 15 tanks were produced by the United States Army. Most were derivatives or foreign designs or very poor quality private designs.. Budget limitations and the low priority given to the Army meant that there were few resources for building tanks.

Oh and by law the US military was not permitted to have Tank divisions. The Defense Act of 1920 restricted tanks to infantry use only; as a result, the Tank Corps was disbanded, with the remaining tanks distributed among the infantry.

So to wrap it up. The US army prior to WWII was not an institution which had much financial support to pursue innovative high reward projects with long research cycles like radar. They did not even have the financial support to pursue proven technologies like torpedoes, fighter planes, and tanks between WWI and WWII. During their massive build up of 1939-41 they were focused on overcoming the logistical chasm the US lack of commitment had dug in proven military weaponry and just providing the basics to a military which was growing 800% in three years from 1939-1941. Even this modest goal they barely had enough political support to accomplish.

They weren't slow to adopt radar. My father was battalion commander of the USAAF Signal Corp unit (largely radar operations) in China during the war, and related that the US Army had begun to establish radar units as early as 1939, not too long after the British had their early radar units operational.

Given that bombs weren't falling on the US, while they were falling on Britain, and that (at least at that time) the US considered itself relatively immune from air attack due to the distance involved, the US didn't have as pressing a reason to fully implement radar in 1940. The adoption in the absence of an immediate threat was surprisingly speedy for the normally conservative US Army.

Ironically, it was Japan who was slow to adopt radar, much to their detriment later in the war.

To fully understand how Pearl Harbor happened, you must consider not just the events, but the context in which they happened, the mindset of the decision makers and military leaders in 1941.

What the US was slow to do, is believe was that anyone could mount a major air attack on US territory in 1941. That is why the radar operators on Hawaii thought the incoming aircraft were US - it wasn't possible for Japan to mount an air attack of that scale from 3000 miles away. One or two unidentified planes might have raised an alarm... Japan did send some individual long range flying boats on nuisance raids to Pearl later on, operating from islands or ship tenders. But a large formation? The US was the only country that could fly a large formation near Hawaii... or so the military leadership thought. No military had ever operated a large group of aircraft over that distance. Not until that day.

The US did think that Japan might attack some of their remote bases in the Philippines or elsewhere, which is why the carriers were gone on Dec 7 - they were delivering aircraft to Wake Island. But Pearl? Too far away.

It is also important to consider just how risky that attack was for Japan. They committed most of their carrier force to an attack on a well armed opponent, with no friendly bases or support within over 2000 miles, meaning that damaged ships would have been lost, and the fleet could be subject to counterattack for the two weeks it would take to return to Japan after the raid.

If the US had detected the fleet and ascertained their intentions, the Japanese fleet could have run into nine battleships, three carriers, and numerous smaller combat ships in mid ocean, and suffered great losses that Japan didn't have the industrial capacity to replace quickly.

This was another reason that military minds in the US didn't consider a naval air attack on Pearl Harbor seriously - it was too risky for an island nation that depended upon it's navy for all military capability, to undertake. That could have gone very badly for Japan.

The Japanese attack on Pearl Harbor was unprecedented in scope and audacity. They took a huge risk, and the fortunes of war smiled on them... briefly.

This was not a matter of not adopting radar. It was a matter of not believing what the radar told them, and not believing it for good reason - most nations would not have done something that risky.

A mindset parallel can be found in the 9/11 attacks. Prior to that, no one would have seriously thought that an attack of that cruelty, and on that scale, could be mounted by a terrorist group.

The failures seem to have been at all levels, but ultimately stemmed from the distribution of responsibility for planning and implementing an effective network.

In hindsight, it is easy to see that there should have been a single authority with that responsibility.

In reality, there wasn't.

In the aftermath of Pearl Harbor, Robert Watson-Watt was called in to give an assessment of what went wrong. His report was damning. He identified:

a multitude of reasons why American radar, radar sites, radar organization, radar planning, and radar operators were not good.

He found that operators were:

"ill-selected, ill-trained, inadequate in numbers, and only transitorily within [the] Western Defense Command"

and that planning at top levels was poor, declaring that:

"the distribution of responsibility for planning between the War Department and the commands was indefinite and illogical "

In his opinion, the higher commands simply:

did not realize the inherent technical limitations of radars.

THE SIGNAL CORPS: THE TEST (December 1941 to July 1943), Washington, 1957, p111

(The book is one in a series. They are available in pdf format and make for fascinating reading)

EDIT: I've carried out some further searches today, and although I found found a number of other references to Robert Watson-Watt's report in secondary and tertiary sources, I wasn't able to find an online copy of the report itself. It is possible that it is still only available as hard-copy in official archives within the US.