Because that scene wasn't the best place to tell Bolt that he didn't have superpowers. They thought that through dramatic failure (i.e. getting his ass whooped) that he would figure it out, but it didn't work with the story line. The scene in the movie where Rhino frees him from the truck and he figures it out then was a much more emotionally introspective place and they went with that.
Because...(and I quote)
"The producers decided to nix this scene, as they not only felt that it was way too dark, but it didn't fit into the story structurally." -IMDb
