A lot of people have pointed out inherent ideological components of this platform, but I would also suggest that the lean is likely in part from network effects.

How did each of us find our way here? Someone likely mentioned the platform on another social site, or linked a meme, or shared other content.

If the site has lots of left leaning content that gets shared by left leaning people in places where such people gather, it’s going to bais the new arrivals in a similar direction. This is true of most social spaces, I think. And it’s good! I want right wingers to hang out in right wing spaces like Twitter, just like I want them to hang out at their own bars and clubs, away from me.