The popular parts of reddit are popular because they appeal to many people, it's the same as music. The most popular bands will have the broadest appeal. So most of the reddit front page is cat pictures and slapstick humour because that's what everyone can appreciate. If the front page was full of obscure stuff about haskell , less people would enjoy it.
In terms of racism, I've never seen racist or sexist content on the front page when I have visited.
I just looked and there's a "Chinese Army Drill" photoshop on the front page. The picture itself is one thing but things have contexts and given that this exists: http://en.wikipedia.org/wiki/Chinese_fire_drill
you gotta wonder where the artist is coming from.
In terms of racism, I've never seen racist or sexist content on the front page when I have visited.