Many, many reasons. Comic Sans was designed in 1994 by Vincent Connare, in an attempt to design a typeface for speech bubbles and dialogue boxes that looked better than Times New Roman, which being a serif typeface more useful for letters and newspapers than interface details. To do this, Connare took inspiration from the lettering of such comic books as Watchmen, being an open aficionado of the works.
For the purposes that it was intended for, Comic Sans is reasonably successful. It's not great by any stretch of the imagination, for reasons I'll explain in a while, but it's better than Times New Roman. The problem arises when it's taken out of those purposeful fields. Comic Sans has many issues which keep it from being a good typeface. First of all, it's childish to the point of ridicule. Secondly, there are notable inconsistencies between various letters, which makes the whole thing look incongruous and tacky. Thirdly, people use it in very inappropriate contexts, ranging from corporate e-mails (which should be done in a serious typeface, along the lines of Avenir or Adobe Garamond or something) to shop signs (even if the "friendly" perspective is desired, there are better sans-serif, rounded typefaces than Comic Sans, such as variants of Helvetica - yes, I know the hipster complaints well enough not to use the typeface myself, but it's an option) and sometimes places where seriousness is absolutely required (curricula vitae should be filled out in a typeface such as Adobe Garamond if in hard copy, or perhaps Georgia if on the screen).
Comic Sans has an appropriate place. Most of the places it is used are not appropriate. In this perspective, it's better than Papyrus, which should simply never be used anywhere, but not much better.
