Or because films have been made for 100+ years all around the world. I think it's not as much going for "intellectual" films as critics having a global vision of the medium they are analysing. Of course older films or non-American films dominate, there's more material from 1898-1970 than 1970-2012 or from the rest of the world combined than just the US.
That's just how critic works. Once you get to judge a piece of art as a part of the canon, you need some perspective. The Beatles, Hendrix or Led Zeppelin are regarded as the greatest because they have proved to be time-proof, which is something you can't say for any modern band or singer - just because you don't have that perspective. Also there's the fact that when you look at the past, there's already a defined list of what's "worth remembering", so most people have listened to Led Zeppelin.