What Disney wants, Disney gets. Nemo is the second-highest grossing Pixar film (and the highest adjusted for inflation). It makes sense for them to produce a sequel.
Surprisingly Cars never did all that well, and it still spawned two sequels. I'm guessing it costs a lot less to animate those, though. And Planes was intended to be direct-to-video, until it tested really well.
I just wish they'd do a sequel to Incredibles. That's the one I actually want!
Global box office stats for Pixar:
1 Toy Story 3 $1,063.2
2 Finding Nemo $880.6
3 Up $731.3
4 The Incredibles $631.4
5 Ratatouille $623.7
6 Cars 2 $559.9
7 Brave $535.4
8 Monsters, Inc. $528.8
9 WALL-E $521.3
10 Toy Story 2 $485.0
11 Cars $462.0
12 A Bug's Life $363.4
13 Toy Story $362.0