Sparticus mentions a good point in that the budget plays a role in it. TV studios don't have the money that studios like Pixar and Dreamworks have. The talent at TV studios also factors in as either they aren't up to the level as feature studios or they don't have as many people working on it or both.
Another reason is time. Feature film studios have around 4 years to work on 1 movie and make it as best as possible when it comes to animation. While TV studios have to turn out lets say 13 half hour episodes within a span of a year. There just isn't enough time and resources to fine tune it to the quality of UP, Shrek, etc.
So basically its the same reasons why 2D tv shows aren't as nice to look at as a traditional Disney film. Hope that answers your question.
Edit:
Forgot to also say that working in 3D can be tricky at times( so can 2D as well ). It requires posing things out in step keys( so just poses no in betweens ), changing poses and composition, putting in linear keys( rough in betweens now ), smoothing the curves out, then changing the timing for overlapping action. It's really time consuming stuff.