Throughout Mists and Warlords, our level 75 defensive talents have been exemplary. The choice between the three talents was always compelling and varied; the universally useful (but minor) Healing Elixirs, the strong damage reduction (but restricted uptime) of Dampen Harm, and the impressive magical damage reduction (but situational use) of Diffuse Magic.
But when the tanking paradigm changed in Legion, these talents suffered. In a world where big defensive cooldowns were neutered, and active mitigation was molded to fill that role, Diffuse Magic and Dampen Harm fell into disuse. Compared to Ironskin Brew, both abilities were ineffective. Diffuse Magic could reduce magic damage pretty well, but why waste a talent on it when Ironskin Brew is nearly as good, and most boss burst magic were designed to be countered by frequently usable active mitigation anyway. Dampen Harm could be used to reduce those giant physical attacks, but if it was used at the same time as Ironskin it was unlikely to even trigger. These two talents were built in a time when infrequent defensive cooldowns were necessary, and clung to that ideology even when tanks changed to value frequent defensives much more.