Another reason is the supporting cast. I love Dick Van Dyke (one of the villains in the first movie), but he didn't have a lot to do and was absent for most of the film. The MVP of Smithsonian has got to be Hank Azaria. Playing the main heavy, Kahmunrah, with an accent borrowed from Boris Karloff along with providing the voices of The Thinker and Abe Lincoln, Azaria really delivers. His scene with a certain green muppet and a black cloaked bad guy is hysterical.
Owen Wilson and Steve Coogan, returning as miniatures brought to life, are very funny as is Bill Hader as a crazed General Custer. I can never tell Amy Adams and Amy Ryan apart but I can assure you one of them is in this movie playing a fast talking Amelia Earhart (I almost said whip-smart, but I can't stand that phrase). And while Jonah Hill might already be overstaying his 15 minutes of fame, his scene here with Stiller is pretty strong.
The only disappointment is the usually dependable Christopher Guest has absolutely nothing to do here as Ivan the Terrible. Oh, and the Jonas Brothers make an appearance but it's brief so no real harm done.
I can't say I'm looking forward to Night at the Museum: Brunch at the Whitney (in 3D), but as long as this one stays in heavy rotation at our house, I can't complain.