Why did Allied invasion of France occur only in 1944 and not earlier?

The Battle of Britain was the same job as Normandy with the same pitfals. Only the direction is different.

For a succesfull opening a front one need air superiority, exceeding manpower, secure supply lines and crippled defenses on the shore.

Britain was defending themselves, the most of air battles happened above their waters and land - the pilots shot down were rescued or captured by british army, british planes have much more air-time than germans - thy didn't need to cross the chanell, germans did have to.

British infrastructure was healing after the Battle of Britain was won with limitted resources for invasion. The Atlantic was not U-boat free so intensive logistics from US were not possible.

In 1944 the Allied forces were able to secure air superiority over France and Europe, they were able to gather enough resources and manpower on the Chanell shore and still the invasion was not easy at all.

Considering invasion in earlier stages it might have lead to Germany holding their positions and causing critical losses to alied forces. Having opend fronts in the East, West and Italy, Germany was overwhelmed and collapsed. Failing the Normandy would lead in longer war hopefully pushing the Germany through entire Europe towards then-neutral Spain or gathering resources for Normandy 2.

All good answers but has anyone thought about the battle of the Atlantic?

You can have all the soldiers you need even at 100/1 but if you can't supply them with food, equipment, ammunition... you get the idea... how far do you think a second front would go.

Britain in 1942 was totally isolated, with very little left except for Roosevelt eventually agreeing to supply the U.K. With what they needed to survive and continue fighting.

There are speculations on when Roosevelt decided, with the rest of the allies, to use Britain as a staging area for an invasion.

You also need to remember that the Americans were at war with Japan and that kept them very busy, especially with their supply lines.

Also worth consider than by 1939, the US army was no better than your local scout group(no offence intended here). It took them a long time to build, train men and organise their army.

The convoys that the American sent were the only source of help for the British.

However, the British won the Battle of Britain, conquering the skies making it safer.

But the u boats did more damaged.

When it was finally safe, Britain was chosen as a staging area. But it took all that time to bring men and equipment over.

There are many sources I use for these statements and it'll be futile to name them all.

Uh, because they couldn't? You make it sound like the Allied forces could just waltz into France whenever they wanted. In 1942 the US had nowhere near the resources necessary to mount an invasion of France. Basic problems like where would water and oil come from were completely unsolved. The Germans had air control. That alone prevented any thought of invasion. The allies were only doing night bombing. Why? Because a daytime raid would have been suicidal. If you cannot even bomb Europe safely, you are not going to be able to invade it safely.

It depends on how you define "Second Front."

The Allies opened a "second front" in North Africa in November, 1942. That was huge because the battle of Stalingrad was going on at the same time. Germany had to divert most of its air transport fleet to reinforcing North Africa, and suffered heavy casualties. When the transports returned to the eastern front, there weren't enough of them to resupply Paulus by air (even though German transports had done the job of supplying surrounded "pockets" near Moscow, in the winter of 1941-42.)

In July, 1943, the Allies invaded Sicily (followed by Italy in September). This diverted some 20 German divisions from the Russian front (almost as many as were lost at Stalingrad), even though they were sorely needed in Russia after the battle of Kursk.

The thrust of your question seems to be, why didn't the Allies open a "second front" in FRANCE earlier than 1944. That is a matter of logistical and other issues. One deterrent was the failure of a "probe" at Dieppe in August, 1942.