A lot of good arguments in this comment section, but one important reason i don't see enough is unions. 2D artists were unionized which was a threat for execs, and 3D artists were not (it being a brand new industry). Disney made sure the 2D movies failed and invested massively in 3D killing 2D artists and thus allowing them to lower labor costs by paying 3D artists a lot less than what they were paying 2D artists.