Arum was a legendary promoter, Manny was under the Top Rank promotion for a long time so Arum really did a great job on making Manny popular on his sport, but maybe it's really the trend that when a boxer is already popular, they create their own promotion so they can maximize their potential income.
Bob Arum has his share of controversies throughout his career as his way of promoting a fighter is that he will be keeping majority of the fighters purse and that way he made a lot of money and there are rumors that he used to fix fights and we know how corrupted the boxing judges when we watch certain fights. Arum and Don King controlled the boxing promotion for a very long time as a monopoly.