it's not necessarily harder, but it allows for a much better division of labor because you can have groups and groups of people dedicated to subtle things like lighting, rigging, texturing, rendering, etc. so, essentially, you can get more people on it because the guy in charge of the lighting doesn't necessarily have to be good at anything else. but, it costs more money to make them as an exchange.
2D movies kind of require a much larger group of highly skilled people who are proficient in all of those things; lighting, anatomy, movement, color, etc.
as a result, though, CGI doesn't have half the charm and expression that 2D does.