Because Liberals tend to join on an open platform where we can be open and learn from another, on a social level.
Majority of social platform users (all troll and idiots aside) are liberal based on the nature of the platforms.
Conservatives tend to be more traditional and stagnate (no stigma intended), and sites like this impede on that. Conservatives by that nature, coming to a site where people are open about their sexuality, encourage progression, and welcome social changes where we could learn, isn't exactly a Conservative podium.
Open source, sharing, and progression is the nature of the majority of social media. Which Liberals such as myself love. Conservatives are actually trying to figure a way to regulate the internet but that's just unrealistic. So again the nature of social media and the Internet, is mostly liberal by nature all politics aside.
I think a lot of it has to do with the average age of the users tend to be fairly young. I once heard someone say that "If you are young and not a liberal, you have no heart. If you are old and not a conservative you have no brain." I think that sums up pretty well why young people are so much more likely to be liberal
There are also the matter that there are a lot of people from other countries on here, many of which are from more liberal cultures.