Senate leaders hoped they would get the necessary votes ultimately, but they were worried at the time and faced financial and political repercussions if the vote cratered in public view.
This is all lying bullshit, of course.
McConnell needed to keep the vote secret until he could hammer a sufficient number of RINOs into following his lead, in order to “give him cover” in his upcoming doomed race for re-election against Matt Bevin. And Harry Reid was more than happy to give him cover. After all, he and Obama got what they wanted out of the whole deal: Total victory, and total surrender on the part of what is now little more than Gentry Oligarch Party.