the problem is that there is usually only 1 attractive keystone for most similar types of champs.
warlords and fervor are two keystones dedicated to auto attackers. a really large group of champions. only one of the two will always be stronger and attractive for them. deathfire grasp is only good on mages with damage over time abilities who stack a ton of ap. so really nieche compared to the whole champion pool.
people have been arguing champions like singed, but considering his health pool and how little ap he stacks overall compared to a mage, he would probably be better off with grasp or bond of stone.
thunderlord is the only attractive mastery for mages and assassins with burst which combined makes another large portion of the champion pool. windspeakers blessing is extreme nieche for heal/shielding supports.
resolve really only has grasp and bond of stone depending if youre a fighter/tank or support.
the problem is, that only 3 keystones really appeal for a majority of the champions, while the rest specifically caters to small groups. or is simply bad (strength of the ages and stormraiders.
and then you have overlap, where champions of one large group like certain marksmen can make use of thunderlord, a mastery that already appeals to another large group.
no mater how the masteries are going to change, the champion pool doesnt equally consist of all archetypes to distribute the keystones evenly. and not all types of champions are picked evenly.
marksmen are in every single game on every side. so naturally fervor or warlords will always have a high pick rate.