There are multiple reasons why the Manny Pacquiao-Floyd Mayweather Jr. fight never happened up until this year. There was the issue of Bob Arum, with whom Mayweather did not want to negotiate. There were concerns from Pacquiao about being blood tested in the days leading up to the fight.
Ultimately, it’s clear in retrospect that Mayweather was fearful of Pacquiao and that is why he found a reason not to do the fight. Pacquiao said on HBO’s “Mayweather/Pacquiao At Last” that the drug testing demands were just an alibi by Floyd’s camp for not wanting to make the fight happen.