2D Fighters are considered more visually appealing(combat wise) due to the craziness that you see happening in most of them, along more ease of access often times. But this just my take on it.
So basically 2D fighters are popular because of the existence of Street Fighter, "visually appealing ala Mahvel", simpler metagame, and one less dimension to worry about (i.e. no environmental positioning, wall-splats, ringouts, etc.). Interesting...
Street Fighter is the reason our game exists. Capcom paved the way for the evolution of fighting games. As much as I love DOA (and I love this fcking game), SF is way more advanced in terms of execution, skill level, and gameplay. Numbers don't lie.