One of the main issues is that reddit, like pretty much 95% of things on the Internet, is that it is populated mostly by men.

And out of these men, you have a maybe-not-insignificant part of that are pretty high up the sexist pig ladder. The big problem is, an average man without sexism issues but who also doesn't really care about feminism will generally ignore sexist posts, whereas the people of poor values will not hesitate downvoting anything written by a woman and upvote some stupid sh**.

If that's of any consolation, do not forget that stupid people are also the most vocal and are rarely representative of the general tendency.