This problem can be disected 100 ways. Mayweather fans have excuses for Mayweather and blame Pacquiao for everything. And Pacquiao fans have excuses for Pacquiao and blame Mayweather for everything.
I believe that both are to blame for this fight not happening. At one time, you didn't really have the option of saying who you were and were not going to fight. They wanted to prove that they were the best boxer. If you were the champ, you fought the number 1 contender, period. Muhammed Ali got punished by Joe Frazier in their first bout. But he wanted to fight him again. Then after he won, both men couldn't wait to fight again, even though they nearly killed each other. After losing and having his jaw broke by Ken Norton, they fought each other again only 6 months later. And after George Foreman destroyed Joe Frazier and everyone warned Ali that Foreman would probably hurt him, Ali still fought him.
That is just one example of how it didn't matter what the circumstances are, the 2 best fighters should fight each other, and more importantly, should want to fight each other. I think Floyd and Manny are both timid about fighting each other. It has been hyped up so much that both men are afraid to face the consequences of losing. It would be a devastating loss and a big hit to thier careers and egos.
Also, Manny is considered by most as the best pound for pound boxer and losing would strip this title. Floyd is undefeated and doesn't want to jeopardize his record.
Floyd has never had his bell rung. The closest he came to this is when Mosely made his knees buckle. He knows that Manny can and probably will connect with some clean, hard shots. And Manny's weakness is good counter-punchers, which is Floyd's specialty.
So I think both of them are scared to fight. Whether Floyd's setting up hurdles and making it nearly impossible to fight, or Manny's hiding behind Arum; I don't think either want the fight.