User blog comment:D. Cello/Assassinews 03/05 -- ACWiki's Fan Section and AC3 first trailer/@comment-182.239.197.102-20120307083538/@comment-3130611-20120307160830

Two main reasons I can think of:

1. The majority of gamers live in America and Europe, and since they've already had 2 games in Europe (3 starring a European ancestor), they set it in America to appeal to their American base. (NOTE: I said "majority", not "all")

2. Like it or not, America has had a significant impact on world history, and many people view America as the first country ruled "by the people". Assassins would support this rule by the people, while the Templars would want to subdue it (which they effectively did through corporations, i.e. Abstergo). Makes sense to put it there.