BMI and ASCAP are collection agencies for songwriters and publishers.
BMI came along after ASCAP and it was formed by broadcasters. The whole
idea behind its formation was to start signing their own writers. When
those writers scored with a hit, BMI no longer had to pay ASCAP. They would
pay their writers directly and less.

To the point...writers who wrote the amazing songs that came out of Tin Pan
Alley all knew each other. Some were given lavish bonuses to sign with BMI,
while some stayed with ASCAP knowing that if they had a hit song the payoff
would be greater.