Our Debate A person did their best with a difficult topic, but at the end of the day their argument was predicated on two hurdles I don't think are all that hurdle-y:
Adding them would inflate the union too much and what to do about their dues.
There are solutions to these problems that don't have to be catastrophic for the union. For example, maybe each minor league team has one representative and as a team they split the union dues. You're adding roughly 150 representatives then, enough to be significant but not detrimental. Or maybe you send two voters. Whatever helps keep the balance from going too far one direction.
They need to be fairly represented, especially since the league's ability to act like a monopoly and control career paths is protected by Congress. Right now they aren't and the only path forward is through some new formation of the union.