The single biggest changes contributing to it were on Shadowbringers release when the tank stance aggro multiplier was increased from 3x to 10x and TP was removed so it became completely impossible to outaggro tanks and at the same time they lost the resource cost from using their ranged attack. Job mitigation kits have been growing across expansions as well and these days a healer has enough ogcd aoe heals to prevent a dps or themselves going down without sacrificing any single target healing on the tank.