Several reasons IMO
Good animators, they sent the animators to mime class to learn about weight distribution (that's why most CG feels weightless), they studied animal movements, and they had stop motion animator guru Phil Tippet as well to guide them.
Darkness, it hides some of the shortcomings of the CG rendering, but still the dinosaurs in broad daylight look pretty damn good. They don't look like they're made of jelly.
Reasonable number of shots, so they had heaps of time to get things absolutely perfect, JP has less than 100 CG shots IIRC. By comparison, StarWars Episode III had about 2000 CG shots.
They put more effort rehearsing the scenes (they used small dinosaur marionettes to plan the shots) so the actor's performances would match up with the CG later. With some recent films, it seems they just improvise on the set and let the animator have a migraine squeezing the CG into the shot.
I miss 80s/90s ILM
http://www.youtube.com/watch?v=TFqJtcRTqZg
http://www.youtube.com/watch?v=miVlDKGa8F4
The result...
Jurassic Park - Before And After
http://www.youtube.com/watch?v=Oz5Asn03qF4