Because the Republican Machine is driven by RINOism and Romney is an uber RINO and Newt isn't. Just that simple.
The RNC knows that should Newt become President he will try and restrain government and trim/cut government programs. That would mean (horror of horrors) a reduction in the RNC's power. Can't have that. By promoting Romney they get a guy that will pretty much leave things the way they are or maybe promote a few more poorly thought out government programs.
Well put