like someone said before, because retro games have an unexplicable quality to them. It's very difficult to give a high res game this quality. It's like asking: why do some people love vintage cars? they were loud, ineffiencient, bulky, and inferior to todays modern sportscars. Well some people just don't like those modern cars, or maybe they do, but they still hold retro in their heart.
I also beleive that people like retro in the 2d gaming scene because it has proven itself ( with releases like metroid,mario, megaman etc.) to be fun, and able to tell a tale with simplicity.
high-res 2d is still a bit more niche, and is only starting to prove its worth (world of goo etc.) as having the ability to create a "great" game with heart and gameplay as the main features.