This is actually a really good question, because it raises the question of how our framing of a historical event shapes our understanding of it. Here's an example: some Southern nationalists object to the term "Civil War" because it implies that it was a conflict that took place within a single country, and therefore delegitimizes the Confederate cause. From this point of view, "War Between the States" or (more provocatively) "the War of Northern Aggression" would be preferable.

The same principle applies to the conflict that took place between 1754 and 1763. The term "French and Indian War" makes sense within the context of United States history, since it distinguishes it from the other war that took place during George II's reign, and identifies the major enemies of the Anglo-American armies. But note that this is an essentially nationalist perspective, in the sense that it adopts the viewpoint of the Thirteen Colonies (and therefore the United-States-in-embryo), and understands the war in terms of how it affected their development.

But of course there are other perspectives on the war: that of the inhabitants of New France (whose descendants often call it the "Guerre de la conquête", or war of conquest), those of the European powers (who were as much concerned with the European or Caribbean theatres of war as with America), the Native Americans who fought alongside both the French and British, the Caribbean islands involved in the war, as well as soldiers and colonists who fought in West Africa and India. So to encompass all these points of view, historians seem to have decided that "Seven Years' War" seems to be more appropriate (even though when you add up all the years of fighting in both hemispheres, it actually turns out to be a nine-year war).

Part of this has to do with the growing emphasis on Atlantic history since the 1980s - a shift away from looking at colonial North America as a precursor to the United States, and toward tracing the imperial connections that bound societies in Africa, America and Europe together. And from an Atlantic perspective, "Seven Years War" seems like a better term (Lawrence Henry Gipson's "the Great War for the Empire" would also do the trick, but it's a bit of a mouthful).

Finally, Fred Anderson's Crucible of War has become the standard work on the conflict since it was published 15 years ago, and Anderson uses the term "Seven Years' War" (he explains the reasoning in his first footnote). As a result, many historians have followed suit.