The Matrix had a much bigger pop culture (and film making) impact than American Beauty. I know that's not the sole criterion or even a paramount criterion for Academy voters, but it should be.
Take last night's winner, Moonlight. (Or the Artist, or most recent winners). I predict that nobody except film buffs will find these movies particularly memorable a few years from now.
Conversely, Michael J. Fox stepped out of a Delorean on stage last night to thunderous applause, over 30 years later. It's still as beloved a film now as it was then.