1) It is overused, since it comes free for most people, and it was the first font people found that wasn't as formal as Times New Roman (default on Word/Office for many years) or stiff and boring like Arial. So people liked it, and used it.
2) It's not really a good font. It's a first-draft font that the designer clearly had not the time to finish properly. I give the benefit of the doubt to the designer, Vincent Connare, here that he knows what a good font makes. Without going into too much detail, it's the uneven kerning and uneven weight distribution that makes it bad. The font never made it into Bob, but the developers of Microsoft 3D Movie Maker liked it, so the font made it into the standard Windows font list.
3) It was intended for replacement in Microsoft Bob for the speech bubbles, which when Connare saw it used Times New Roman. Which is a badly used font, too, even worse than using Comic Sans for longer texts, as far as I am concerned. Connare let himself be inspired by comic book/graphic novel lettering, but he did not fully understand comic lettering. So today one should not even use it for comic lettering if one wants to appear professional. For instance, comic lettering is usually caps only, so professional comic book fonts use the shifted/non-shifted letters for all caps letters with slight variations, to emulate hand lettering, and fulfill certain typography rules, such as using an "I" with crossbars for the personal pronoun only, and just a vertical stroke for all other "i"s. Comic Sans includes standard upper/lower case letters.
4) There are way better fonts out there (some even free) that do not have these issues, but to the untrained persons look very similar to Comic Sans.
There you have it. It's a quick shot font not fully designed with very limited usefulness that by the powers that be was found in a position to be used for text it was massively bad for by people who had no idea why, while better alternatives are available. It also didn't help that Microsoft themselves didn't know and wasn't able to include guidelines into their products (not only for font usage).