Why did German U-boats not sink American ships carrying soldiers during WW1?

due to the limitations of battery life and the fact that a uboat needed to surface to recharge them and to actually see the prey tactics soon were developed by the royal navy that made a nightmare on a uboat, Also American industrial output made it virtually impossible for The Kreigs marine to win the war in the Atlantic

They did, in both wars.

A primary reason that US troops weren't lost in transatlantic transit was the use of high speed passenger liners as troop transports.

Those ships, expressly designed for high transatlantic speed, could travel at speeds of 20-25 knots, whereas the WW1 vintage cargo ship was 6-9 knots.

The typical German WW1 Type UA and UB submarines had a submerged top speed of around 5 knots with very short endurance at that speed, and 12-14 knots on the surface. A passenger liner could easily turn away and outrun a submarine if it was on the surface. Submerged, a submarine couldn't possibly position itself for an attack with the liner traveling at top speed, short of being incredibly lucky to be in the right place, and none ever were in that right place.

For this reason, troop ships did not sail with a convoy. They traveled alone and away from normal shipping lanes, using their high speed as defense. In convoy, they would have been restricted to the speed of the slowest ship, making them an easy target.

And, this worked. During WW1, only two major Allied passenger liners were sunk at sea, the Lusitania (torpedo), and the Britannic (mine).

The Lusitania was traveling slowly off the coast of Ireland due to nearby fog when hit, and was in standard passenger service at that time, so it was not using its top speed.

Britannic was a hospital ship en route to the Dardanelles when it struck a mine, so it had just the crew and medical staff on board, and loss of life was very low.

The Lusitania's sister ship, the Mauretania, was used as a troop transport in WW1, and was never successfully attacked.

They did. Or rather, they tried…?

"Unrestricted submarine warfare" meant just that: target every enemy ship, or ships you think that are (?), and try to sink it. After the American entry into the war the orders were changed from "try to avoid American ships" to "burn them all".

Of 7,283 ships attacked, 174 were American.

Things did not change dramatically upon the American declaration of war. In some respects, the German High Command was correct, and it would take between twelve to fifteen months for the American military to make a real impact on the battlefield. In other respects, they got it wrong. US industrial and economic might was unleashed, and could build and launch merchant ships and destroyers faster than German submarines could sink them. As these same submarines began sinking ships indiscriminately, they sank ships belonging to other neutrals such as Brazil, who subsequently joined the war, sending troops to fight in the trenches as well.

Having reluctantly agreed to the 1917 round of unrestricted submarine warfare, and having failed to keep the United States out of the war, Bethmann resigned his position as Chancellor in July 1917. This act sealed the victory of Hindenburg, Ludendor , and the military over the political wing of the German government. It also served as a reminder to Wilson and House that Germany had fallen from the civilized nations into a nation of warmongers.

_{Justin Quinn Olmstead: "The United States’ Entry into the First World War. The Role of British and German Diplomacy", Boydell Press: Woodbridge, Rochester, 2018, p 158.}

Specifically:

The next day, 15 August 1918, U-117 resumed her mine laying operations off Fenwick Island Light. That field later claimed two victims, one damaged and the other sunk. On 29 September 1918, Minnesota struck one of those mines and suffered extensive damage. The Naval Overseas Transportation Service cargo ship Saetia entered the same field on 9 November, struck a mine, and sank.

Also look at SM U-140, SM U-151 and the lists of shipwrecks in 1917 and 1918.

As said above: they tried, and they tried hard. Very hard. After all, that was part of the contingency planning should the US do the for the OHL unthinkable, try to bring actual American troops to the battlefields.

But with their methods and vulnerabilities becoming known, the elements of surprise ensuring either victory or at least escape got fewer and fewer.

Not in the least because of a successful adaption: the Convoys in World War I:

Between May 1917 and the end of the war on 11 November 1918, only 154 of 16,539 vessels convoyed across the Atlantic had been sunk, of which 16 were lost through the natural perils of sea travel and a further 36 because they were stragglers.