"Because the players from the 80s and 90s would never stand in the box without a helmet on."

There's a lot to this. As a Steelers fan, I couldn't imagine the incoming class going up against the Steel Curtain from the 70's. No way. They may be bigger and faster, but looking at a guy who proudly had no teeth and slammed the opposing defender to the ground for simply disrespecting his teammate?

There's a lot to be said about this idea. Let alone when there was basically NO protection at all in either sport but a leather hat

As brush mentions above, the 20's & 30's didn't have non white players. That's a big one, all by itself.
They were all hard drinkin' and partyin' men, who thought nothing of lighting up a cigarette in the dugout before they went up to bat or after, lol. They didn't work out or take particularly good care of their bodies.
But one of the biggest reasons IMO, is what the players of the 80's and 90's had, that the 20's and 30's didn't: Greg Maddux.

Okay, not just Mad Dog, but him and players like him. He just illustrates it well (and all while looking like an accountant who would trip over his own shoelaces).
One of Greg Maddux superpowers as a pitcher was an encyclopedic knowledge of the game. He knew every batter who came to the plate and their batting history. Every pitcher he opposed. Every manager. And all the players of the past, and all the plays. AND how to make the ball go wherever he wanted at whatever speed(s) he wanted. Yea, he was amazing. Back to my point...
He wasn't and isn't the only player who is all geeky about the game that way either, as any baseball fan knows (and often shares). So the players from the 80's and 90's are going to have built in knowledge base of the players and the games from the 20's and 30's that will not be reciprocal, because history.
In my humble opinion, this and the considerable advantage gained by non white players pretty much guarantees the 20's and 30's players get creamed by the 80's and 90's players.