Probably because Healers, like Tanks, are basically premium roles. People are far less picky with them than they are DPS specs, and for good reason. Try playing a Necrolord Hunter, any spec of your choice, see how many groups pass you over, and how much stronger a Night Fae Hunter of the same spec and the same gear as you is. The disparity between covenant abilities and soulbinds can be so incredibly high that it’s perfectly understandable when someone gets benched for being the wrong covenant.
And yes, “player made walls” is definitely a limiting factor because everyone knows, including blizzard, that people will choose the path of least resistance, and that means choosing the meta classes, specs, talents, covenants, etc