You can agree or disagree, but I think the logic is that 1900 (or 1901) are considered the start of Modern baseball.
The rules changed a LOT from 1877 to 1903 (Foul strike rule adopted by the AL).
In 1884, Sweeney's year. Six balls were a walk. There was a pitcher's box. HBP didn't get to go to first. They pitched from 50 feet and didn't have to have a foot on the rubber. Foul bunts weren't strikes, there was no infield fly, catcher's didn't squat behind the plate. Bats could have a flat side from 85-93. until 1886 batters could "call" their pitch
It wasn't until 1884 that the National League voted to lift the ban on overhand pitching. The American League allowed pitchers to use a modified sidearm in 1884, as long as their hand didn't raise above the shoulder during release, but then lifted all restrictions on pitching in 1885.
I get the argument that some of these guys are in the HOF, but the logic is because of all these rule changes. Might have been better to set it to 01 or 03. I think they just chose the round number that was close to those magic dates.
https://www.baseball-almanac.com/rulechng.shtml