Because it is a whimsical, childish looking font that was originally designed for readability without anti-aliasing. As a font, its balanced in such a way that it is more readable on a computer using, for example, Windows 95, and imparts a sense of whimsy into short statements. However, we don't use Windows 95, and it has become the font of choice for large paragraphs, and even relatively serious signs. To compound this problem, it ended up being used mainly in messaging clients being used by grade schoolers and similar juvenile projects, further cementing its position. The hate for Comic Sans comes from its widespread usage in terribly inappropriate places.
http://www.kadavy.net/blog/posts/why-you-hate-comic-sans/
That's a very good analysis