ELI5 Why has it taken so long for Mayweather and Pacquiao to fight?

Just don't get why this hasn't happened at all in the past five years? Was Floyd afraid of losing his perfect record? via /r/sports http://www.reddit.com/r/sports/comments/2wsyq8/eli5_why_has_it_taken_so_long_for_mayweather_and/