Stalin (quite understandably) had demanded that the Allies open a Second Front as early as 1942, yet this did not happen till June 1944. Why did it have to wait till so late?
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 June, 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.
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.