Largely because the United States was still trying to get some cooperation out of Vichy France and de Gaulle was viewed as a relatively minor player politically in 1942 - and very certainly not anyone who held any influence over the French government.

The existence of Vichy, their deeply antagonistic relationship with de Gaulle, and the relationship of both with the Allies and especially the American government are massively complex topics. We've had a few answers in the past that can provide a bit of context to this: an ancient but still worthwhile one from /u/run85 here, another old one from /u/Aleksx000 here, a very good answer on de Gaulle's development from /u/oddlyalive here, a comment on the relationship between Petain and de Gaulle by /u/littlecow888 here, and after you read all that, a very worthwhile answer on the scholarship and its biases by /u/Cobra_D here.

What I would say in summary of all that for the purposes of your question is that when the United Nations was founded as the formal term for the Allies on New Year's Day 1942, about the only one of them that had any small influence with Vichy France was the United States. Vichy had broken off relations with Britain after the former had sunk a good part of the French Navy off Oran in July 1940, the Soviet Union had done so after July 1941, and thus whatever small leverage was left to try to limit collaboration came from the United States.

This showed up most prominently in the decision by FDR to send Admiral William Leahy to Vichy as his ambassador; before appointing him just after the 1940 election, he privately told him outright that when war broke out, Leahy was going to be back in Washington running it for him, which he indeed did as his chief of staff.

Leahy's 18 months as ambassador are controversial; a summary is that he got very little cooperation from France proper, but probably made a small amount of difference in some of the colonies before he came back to the US in July 1942. But during that period, FDR not only found little to admire in the leadership of Vichy but also saw de Gaulle insufferable and often in direct opposition to his foreign policy goals. This helped in the creation the utter mess of leadership that took place during the single most important foreign policy goal of the Allies: keeping Vichy forces somewhat neutral during the invasion of its territory in November 1942 with Torch. The Allies went from Giraud to Darlan back to Giraud to a bit of de Gaulle, and over the next 2 years de Gaulle largely took over the government-in-exile as the Free French.

As to why the de Gaulle and the Free French were invited to sign in 1944, the United States broke off relations with Vichy when the Germans occupied it following Torch; there was no point in continuing to deal with a state that had been largely removed from existence. Instead, the Free French could indeed contribute something militarily during the invasion and reestablishment of a friendly government following liberation. Just as importantly, signing on as a member of the wartime United Nations in 1944 was a requirement to participate in the San Francisco Conference in 1945 that formed the United Nations we know today, and France was always intended to be a charter member of that organization.

So it basically boiled down to in 1942 not making a political statement for someone you didn't particularly want to support anyway (which also almost certainly would have led to the breaking off of relations with the French government on the ground then) versus in 1944 recognizing some variety of the government that was going to be reestablished following liberation that the Allies needed to work with in some form after the War.