I don't think it's obvious that people voting on old movies haven't seen new movies.
It might be likely that people who watch many old movies are less likely to have seen new movies, but it could just be that these people watch more movies in general.
It's obvious not because of any character trait that those people have, but because the majority of the voting on those movies was done before the new movies came out.
IMDB top 250 list is dominated by old movies, which conflicts with my perception (shared by majority of people as far as I can tell) that new movies are far better than old movies (comparing either top with top or average with average).
I have a simple theory why IMDB is wrong: