It's mostly so that he doesn't dominate Takedown and Heist. Before the damage buff, where would you play Bull? Takedown, Heist, maybe Siege or Brawl Ball.
If you watch KairosTime, the video on Takedown Bull used to have a 6x higher use rate and higher win rate in Takedown than all other brawlers beside Colt who had a 10x higher use rate. (data taken by KTYT from brawlstats which pulls data from the game )