Because some people are sadistic jerks who love to lash out at other people online, because they are bullies. Bullies are really cowards and what they feel brave enough to spew out viciously on the internet in an attempt to humiliate you, they most likely would not dare say to your face in person. Or even if they would say it to your face, it doesn't change the fact that they're abusive scum. I get flamed all the time on forums of the internet when I am not even trolling, looking for a fight, starting trouble, or anything. Some people just feel temporarily more "powerful" or "superior" or something, by berating somebody else.
They know you can't reach through the screen and slap them, and they bank on the thought that it would be too much trouble for those they insult to track them down and sue them for harassment. So they go for it. The internet is like a haven for them to pick on people and create drama, where they can feed their addiction of trying to tear somebody down. Be careful, if you give hell right back to them which is natural to want to do, they may report you and get your account or email deleted/shut down because often times a flame is a bait post, hoping they'll make you blow your cool and start cussing at them so they can tattle on you to the moderators or to your ISP or email service.
Just be glad you don't have to live with them in real life or work in the same building they do. It could be worse.
By the way, did you say something about Michelle Phan?
It is possible your comment was misinterpreted and they thought you were trying to be mean even though you weren't.