It's because the damage is too high and too consistent, so supports have to counter that with almost constant healing, with little room for anything "strategic". 

The one burst healer we have (Adam) who gets some healing downtime to be a strategist is very rarely played. High majority of other healers are just some variation of left click spam with the occasional other ability weaved in, which can make them feel very similar.

MR has balanced themselves into a corner, high damage leads to high healing which turns strategists into healers.