My 2-cents: For the legends - Ruth, Gehrig, Mays, Cobb, Mantle, Jackie, etc. their value mainly increases since awareness of their exploits continues from generation to generation.
For others, it takes a story/movie to bring them into the consciousness of current collectors. I haven't tracked it, but I wonder if the
2018 movie about Moe Berg led to an increase in the value of his cards.
Some HoF-ers just don't have the appeal of others. Consider Al Kaline, who according to recent eBay sales, has autos that go for <$10. So for some they might be over-exposed among current cards, and therefore their allure suffers from a lack of overall card rarity (even if their playing-day cards have a relatively low pop or are in a desirable set).
Just being in the HoF is no longer what it once was. Consider that Tris Speaker (#9 in career overall WAR) was not a first-ballot HoF-er, or that it took Eddie Collins 4 votes to be inducted. The recent induction of Harold Baines seems to have dropped the standard quite a bit.
But many of the players mentioned in this thread are legends, under appreciated by the overall market as they may be. Clearly they are not under appreciated here on Net54!