I think it is a combination of the small market for them as well as the seemingly large availability in the past two years or so. I have been collecting Negro League items since the 1980's and don't recall seeing Puerto Rican and Mexican League contracts but once every few years if lucky. Nowadays it's as if a major auction isn't complete with one or two of these things.
Negro League contracts seem to be much harder to find.
Another issue may be that the signatures of the player contracts offered aren't that uncommon. If a Chino Smith, Lefty Brown or Josh Gibson came to auction, that would be one thing, but signatures of Leon Day, Willard Brown, Cool Papa Bell and some of the other big guys aren't that hard to find, and can be had on something more visually attractive (to some) than a player contract, leaving these to the guys who collect items signed during a player's playing days. So this considerably narrows down an already narrow field of collecting.
And though important to baseball history, the Mexican, Cuban and Puerto Rican Leagues don't generate the same excitement that the Negro National League, Eastern Colored League, Negro American League, etc has with many American collectors.
If a Satchel Paige 1936 Pittsburgh Crawfords contract was located and put up against a 1939 Guayama Brujos Puerto Rican Winter League contract, I'd bet the Negro League one will beat it every time.
So I believe there is no easy answer, but a combination of them.
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