Nadler did it because he can and because he knew it would piss off the Republicans.
That, and Nancy might need time to whip. They might not have the votes.
The more this clown show is exposed, the fewer of the “almost rational” democrats will want anything to do with it.
Also a good possibility he did it in hopes of stemming the tide of defecting Dems. As this drags on the Democrat party is taking a beating in the polls and Trumps numbers are going up which makes more Dems think about their futures come November. Just saying that this could be a consideration too.