I think the problem is healers never really come out of training mode. The opportunity to use your entire kit is few and far between and devs are too scared to do really big hits. This causes hiealing for the most part to be rather dull and less engaging when things are going right and more engaging when things go wrong.

Content design is set to where things rarely go wrong in casual content which is where healers need help the most. However their gameplay loop is tuned to 8 man savages where things go wrong in early prog which results in a simplistic DPS kit of 2 buttons.