The vitriol against Sarkeesian? I'd guess, more than anything, it's because she sort of became a figurehead of the culture war. Her feminism is pretty run-of-the-mill, her points often rather obvious. She's good looking and mild, her videos are decent but not particularly entertaining or well made. It's rather amazing that she gained any sort of fame outside of a small circle, and I'd attribute that as much to her detractors as anything else. I don't know whether she wanted the controversy, but I'd guess it worked out pretty well for her.
Also, I've heard (over and over) that she misrepresents the games she's discussing, and claims about her raising large sums of cash -- without her products reflecting the sums she's raised -- do seem to me to have some validity. Then, at the height of her popularity/infamy, she made a big deal about her own victimization, deleting comments and cancelling appearances and such, with reference to insults and death threats, etc. Even though that sort of thing happens to all such controversial figures, Sarkeesian was given this special, sanctified status (I remember a friend of mine referring to her as a "hero"). That rubbed some folks, myself included, the wrong way.
In short, Sarkeesian was at the visible forefront of the ongoing movement to inject progressive politics into video game culture. Whatever her other virtues or sins, this was always going to earn her some amount of hatred.