Well, yeah...why not? is it because healer players are not as skilled as dps players hence wouldnt be able to heal endcontent anymore if it were progressively harder?
Or is it because people responsible for balancing damage taken/healing done have no idea how to do it?
Most basic example to explain reason would be; healer balance is not something like "healing is still possible/easy even if we reduce heal amount by X%" because it is not for 90% of healers in our player base.
We can't "balance" certain class groups easily if it is causing us to lose majority of affected players in the end. Class balance for any class can't be objective for every player for this reason as it is considering majority of player pool in end game content instead of a certain group.
Greetings