The Senate, which considers itself more deliberative than the House, has had a standing rule (
the cloture rule) for nearly 100 years that requires a 3/5 vote (60 votes) to end debate on a pending bill. It is essentially a filibuster, which historically was one Senator speaking continuously in an attempt to block a bill.
The "nuclear" option threatened by the Repubs to counter Dem calls for cloture votes on judicial nominees would have been a clear violation of the Senate rules and effectively ended an accepted parliamentary procedure that has, for the most part, worked well for the last century.