Two main reasons that I can guess:
1. Many people simply find it easier*. <img src="smileys/smiley17.gif" border="0" align="middle" />
2. Many people grew up playing games like that, so that's what they want to make.
(*I wouldn't say it is, but it's at least more accessible**. It's comparable to building something out of toothpicks or something with huge construction equipment, which one's easier to pick up?)
(**Despite being more accessible, it's just as difficult to create GOOD pixel art as it is good any other kind of art.)