Those Resistance fights are still there in TBC though, but just like in Classic Vanilla, people have just chosen to ignore them so far, maybe some of the SSC fights will be different, but as of right now theres High King Maulgar, where the Warlock tank is meant to have Shadow Res, and the Mage tank is meant to have Fire res. there are craftable cloth sets just like the Naxx Frost Resist gear, but with arcane resist for Kara.
But on the topic of Classic vs retail, it was simply because from a gameplay perspective RNG = Bad, and Resistance = RNG. same reason hit caps and expertise was removed
the fact that Resist is inherently RNG is a little silly, if u make a fight where u need nature res, you could come with the appropiate Nature res gear, and still die due to bad RNG, if you tune down the damage so this doesnt happen, the resist gear becomes pointless, which means the only way to make resistance relevant is to have it be CC/Utility related in said fight (A sleep, a Silence, etc) but that sounds like giga toxic boss design ngl. can't really find a good way to implement it into the game in a way where it remains relevant, and I Imagine Blizzard felt the same?