Yea I don't really get this either. Some will disagree with my generalization, but from ARR to up to early-Stromblood I've always raided with PLD MT. IMO they were always the defacto MT, so if anything PLD is simply sharing the spotlight now. Early on in Stormblood my PLD co-tank was playing MT until we found how much better it was for me to WAR MT. Even then we shared responsibilities. I fully main tanked o1s and o4s, but since we had already established CD rotation and positioning in o2s and o3s I pulled and then we just swapped after openers.

At any rate, I believe it's more of a player trend than something the devs were consciously aware of. The top-end players will pick whichever is the most efficient and it will trickle down to the community from there, doesn't really matter what the devs do.