2D games have been around since the beginning of video games development and it wasn't until around the mid 90's that 3D games became more popular in the video game industry, this could be attributed to the exponential improvement in technology making it easier to develop 3D games. Because of this the majority of games are now developed in 3D while smaller indie games still chose to program and develop games in 2D because it is easier to program 2D games which normally require more simple controls. Although 3D games rule the video game industry 2D games are still relevant for games with specific and stylised art style and because of that 2D games still thrive in the video game industry even though its not as evident because of the number of 3D games compared to 2D games.
http://www.screwattack.com/news/2d-vs-3d-which-better
http://www.screwattack.com/news/2d-vs-3d-which-better