1) because Dems might ram this thing without explaining to public what's in it.
2) if they did that, Rubio is screwed whether the bill passes or not.
3) if the bill passes, people will find out about the awful parts of the bill later, and the base will blame the "Gang of 8" for capitulation.
4) if the bill fails to pass congress, Obama will step in and pass the "bipartisan" immigration bill that rubio co-sponspored by Executive Order. Which means, if he runs for president in 2016, he ends up in Mitt Romney's situation with Obamacare/Romneycare, since he cannot go hard against Obamnesty EO (identical to "Gangof8" plan) without looking inconsistent or hypocritical.