The majority leader of the Senate has the power to change the rules like Harry Reid did for federal judges and McConnell also did for Supreme Court judges.
The majority leader does not have the power to change Senate rules unilaterally. Harry Reid didn’t change the rules, and neither did McConnell. A majority of the Senate voted to change the rules, each time.
That power to change the rules belongs to the Senate, by majority vote. If McConnell doesn’t have 51 votes, he can’t change the rules. I guarantee there were at least 4 Republican senators who would have voted against removing the filibuster.