Arum never wanted the fight... At least until Pacquiao had a foot out of the door (which 2013 is reported to be). The fight didn't make business sense for Arum; his bottom line is and has been of main importance from the beginning.
Are people forgetting that Al Haymon is still Mayweather's advisor/manager?
Are people forgetting that Haymon and Mayweather dictated the terms that were negotiated, and GBP just presented them at the negotiating table?
When is the last time an Arum fighter's fought a Haymon fighter?
Mayweather leaving GBP just coincides with Arum's timetable of when HE wanted the fight. It is amazing how this situation played out almost exactly like he wanted. He successfully stalled the fight, changed public perception to think it was Mayweather that was preventing the fight, made millions in the process, and in Pacquiao's final year, he's going to make the fight while cutting out one of his main competitors; him saying "See? It was GBP holding up the fight all along" is like a cherry on top.