There have been several reasons why Floyd Mayweather and Manny Pacquiao have not fought each other over the years. One of the main reasons is disagreements over terms, including purse splits, drug testing protocols, and other logistical details. These disagreements have prevented the two fighters from reaching an agreement to face each other in the ring.
One reason is that both fighters have had different career paths and priorities. Floyd Mayweather has retired and un-retired multiple times, and has been selective about his opponents. Manny Pacquiao has continued to fight and has taken on a more active schedule, facing a number of different opponents in different weight classes.
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