I think it goes back to the days when the baseball manger/coach was often just another player on the team similiar to what today would be considered a "team captain" in football. Or was an ex-player who had gotten to old to play. He just kept wearing the uniform, maybe even still technially a player that could play on the roster, just didn't cause he knew there were others who were better than him.
Then because of TRADITION it stayed that way.
Football, hockey and basketball coaches traditionally dressed in suits.
Yes kind of like that. I read an article about it the other day and was wondering if anyone else knew. I had always thought it was in the rules but it's actually not. I kind of wonder if one day anyone will challenge the traditon and dress differently ... maybe in something similar to football coaches?