Partly because I feel like this is such an easy target, and people who have just learned 2 or 3 things about typography are very keen on bashing Comic Sans to show off their newly acquired knowledge.
Partly because, let's face it, it's not that bad. What it does, it does well, it's pretty legible, no glyph stands out from the general harmony, etc.
There's a reason it became massively used. It was included in windows because it was part of Microsoft Chat, which back then was heavily comic book styled, with speech bubbles for messages etc (something that has since then become a standard in IM). It appeared in the font drop-down list, and people picked it when they wanted their message to look a bit informal, not too severe. Of course, someone with design knowledge is able to achieve this without resorting to this specific font (or even a funny font), but most people lack this knowledge.