Mathews and Mullane ran up their numbers in lesser leagues while McCormick's value is almost entirely from the NL. A lot of Mathews wins are from the National Association (1871-75) which is
not recognized as a major league (probably it should be....
https://www.thenationalpastimemuseum...-major-league/
Mullane stared in the American Association which was generally inferior to the National League. There is just one player in the HOF who primarily played in the American Association, Bid McPhee.
Also, as noted above, McCormick seems to have been regarded as one of the best pitchers of his time I don't think that's true of Mathews at all. Not so sure about Mullane either.
Mullane also gets docked for a story (perhaps apocryphal) about mistreating Moses Fleetwood Walker who was briefly his catcher. There is also a story that he pitched with both hands -- so he was a "switch-pitcher."
Glasscock (what a name !) would be the only SS in the Hall of Fame from the 1880s era. George Wright was selected to the HOF but as a pioneer and his best years were the 1870's (he was probably really good in the late 1860s too FWIW). The next SS was Jennings (and then G. Davis and Wagner) who made his mark in the 1890s.