Rubio wasn't necessary to reform passing. But he inserted himself into the process, attempting to position himself as the indispensable person. But his role has mainly been to attempt to slow down the process, find points of disagreements where few if any existed and continue to make sure he had as many escape hatches as possible if it became necessary for him to bail out of the process. Through the whole process it's been pretty clear -- if only because of his refusal to state a clear position -- that there aren't really any policy issues Rubio is focused on. The whole game is Rubio 2016. And thus reform is now hostage to his best guess at what's most advantageous for his future.
The generous interpretation is that Rubio wants to remain the permanent skeptic within the process to allow him to bring over Tea Party support. The less generous take is that he keeps throwing up potential obstacles, concerns and complaints because he wants to make sure he's got as many escape hatches as possible when things get dicey over the next couple months.
I lean toward the second interpretation. Our immigration reporter Benjy Sarlin, who's following this whole massive sprawling debate as close as anyone and is doing amazing reporting on it, leans to the first.
The truth is that the same behavior probably accomplishes both goals. So Rubio himself may not have decided. There may be no answer. Benjy's got a piece here looking at Rubio's constant search for new escape hatches on the path to reform.
But the real point is that Rubio isn't the one who can make an immigration reform deal. It was going to happen without him. What he can do is, potentially, make the path to reform smoother for the GOP. But he's also upped the chance that the train jumps off the chance entirely because of how he's inserted himself into the process.