That chemical reanimated everything in the warehouse including spilt dogs, butterflies, and cadavers but not the skeletons. How come? And we know that skeletons can come to life in this movie because of that one that came out of the ground.
There was no organic matter left in them, they were chemically treated and sanitized. All that was left to them would be the calcium and other elements making up the bones; even the bone marrow probably would have been destroyed.
That's the only reason why I would think that they wouldn't reanimate like the skeletons in the graveyard.