That scourge to boxing jose sulaiman said the drop to 12 rds instead of 15 was based on scientific evidence that more injuries took place in those last 3 rds and they wanted to prevent that, but the real reason they did it is because the television stations that put up all the money wanted to fit a scheduled show into hour-by-hour time slots, and avoid dead air.
12 rds is 47 minutes start to finish including rest periods, plus there's time for introductions.
15 rds is 59 minutes including rest periods, but the intros take you over the allotted time.
The preliminaries fill the 1st hour.

15 rds leveled the playing field for pressure fighters: guys that were tough, rugged, with endurance, but didn't have all the genetic gifts of speed, coordination, reflexes that many top fighters have like a prime Roy Jones for example. Nowadays pressure fighters like Glen Johnson or Andrade get outpointed for the first 8 or 9 rds, but they're working the body throughout doing some damage each rd. By rd 12, they're in total control, and finally have the guy exactly where they want him, and boom, the fight's over! They run out of time. Lost the battle, but won the war. Fighters used to say 15 rds separates the men from the boys.

So many classic fights became classics in the 13th rd.