Because I thought I had something valuable to say.
Maybe I did, and perhaps I didn't.
But about 10 comments later, the conversation (which was related to feminism and casual sex) had devolved to: "You cannot hold X opinion (and conversely must hold Y opinion) or you are bigoted and harmful."
X opinion is one that I largely share. Y opinion is valid but lacking, so far as I can see. I was very glad that someone not me got targeted for publicly voicing X opinion, which allowed me to be. . . well, somewhat amused that a liberal space would be so intolerant. I get that there are certain beliefs -- say, white supremacist ones -- that are not to be "tolerated" under any circumstances. But the issue was not such a clear-cut one.