In the early days of a sport or any new group, the variety of players and recruits is vast, a sort of genetic algorithm for excellence. As the field progresses, the best become the standard and new recruits are judged by those criteria. The game itself starts to change as the players settle into standard "best practices" and eventually it arrives at a particular form.

You see this with something like Mixed Martial Arts. In the early days, it was karate versus sumo versus wrestling versus capoeira -- all styles, all weight classes mixing freely. Over time, the best combinations and skillsets emerged and the styles and bodytypes that didn't fit were phased out by natural selection. Thus we arrived at modern MMA which is basically a fusion of striking, wrestling and jiu-jitsu. The field narrowed.

The OSS was the same. Before they turned into the CIA, they would recruit smugglers and border-runners and all manner of breakbulk roughnecks. Over time, they shifted to only recruiting college-educated Ivy League international relations and Russian linguistics students because that's what the Cold War demanded.

I suspect the recruiting practices of baseball were the same. Recruit everyone at first then pare down as best practices manifest.

Then again, deafness shouldn't really interfere with a baseball player's ability sooooo ... Idunno. Maybe fewer deaf people are striving to get in these days? ¯_(ツ)_/¯