If some senators were against this, why didn't they just use the filibuster to prevent the rules from changing?
Majority wishes to pass a bill that the minority doesn't agree with. Majority wants to change the rules on that vote to get it passed despite minority.
Minority says they can't because the Senate rules forbid it.
Majority changes rules from forbidding it to allowing it.
Granted, that's a simplistic way of looking at it. It would seem to me that rule changes should have higher requirements than simple majority because then the majority can change the rules whenever they wish.
It's common for these to be passed unanimously.
This utilizes the majorities ability in any parliamentary body to set the precedent or rule on a point of order. hence the 'nuclear' option.