Because in the end, most if not all support skills weren't used for actual support but to enable more dps. During Stormblood, this came to a head specifically for Tank Meta, where it expected for dps/healers to manage their own enmity so the Tanks could perform at optimal. While there were stories of tank meltdowns in Savage, it was by FAR the worst in casual content, where if you grabbed aggro from a tank (or got to yellow at least) you could expect to keep it or occasionally get a shirk for your efforts.

This was the most extreme and prevalent example, but it's the reason why most cross-skill support was removed. -Even the niche skills like apocatostasis (sp) which was basically magic damage mitigation skill and whatever the mp gift skill was called for BLM. These were strictly situational but still got the axe. The roles became more focused and homogenized. Healers lost a lot of what made them unique and Tanks lost their ability to fine tune their damage/enmity control. It was an effort to define the roles as something not just dps but in the end it just made it a mindless button mash for the most part.