Because some ministers support the bill, Netanyahu is now risking his image by threatening to fire them, which means some may absent themselves from the vote, or even quit the coalition altogether.
It's a shame that he's going out of his in desperation to please outside sources hostile to Israel, to say nothing of setting a very bad example for a politician by clamping down on their democratic right to choose where they stand. He's also causing some valid worries of another "disengagement" taking place. And above all, I would argue that long-term, there is a possibility that it could damage his chances in the next election the coming year.
Update: here's news about a rally in favor of the legislation that took place.