I think it has multiple reasons.
Like a comment below has also said: Men are way easier to befriend. Especially at gatherings like parties, women tend to act more as though you're competition, or you're simply not what they're looking for. Some women are obviously very welcoming and nice, but most of them tend to turn up their nose. That's at least my own experience.
A lot of my hobbies and interests are also dominated by men, so that definitely works as another factor.
For me, another factor that's actually quite sad is that I've often been used as a trump card for/at parties. When I started going out again, I got invited by my brother's friend to join a party, and I went with her. Most of the people there were guys. I'm pretty easy to talk to, so a lot of them enjoyed me being there. The girl I went with often used me as an entrance pass to get into those parties, if that makes sense. It kind of led me to surround myself with a lot of guys, and I ended up befriending a lot of them.