This goes all the way back to the beginning of the game. In 1993, there was no official website for Magic. The internet was in its infancy. This new trading card game was all the rage. Card spoilers weren't a thing. There were no card information databases. People wanted a way to concisely communicate about this new game. There wasn't cool graphics or emojis, just typed text. B for Black and U for blue developed very quickly as the accepted shorthand for typed mana symbols. At this point, you've got 25 years of inertia behind it. There's no changing it now.