Tanks are already popular enough. It's already hard enough to find a guild as a tank, because there are too few spots and too many tanks around. Check this poor fella:
https://www.mmo-champion.com/threads...n-a-dire-state
Half the answers in that thread were "you won't find a guild, because you're a prot warrior". So it's not just my opinion.
Now healers that's another story, it's really hard to find a competent healer these days for example for a pug mythic+. Too many of them queuing are really just dps players who think healing is no big deal and faster queue, and then fail miserably. Same story on the more casual end, I went to lfr on my rogue and tanks were dying - check healing meters, two holy palas below the tanks in healing done. And can't say there was nothing to heal so they ended up low, because well we wiped due to tank deaths (and no, they weren't in dps spec, they were in healer spec).
Healing is notorious for being to worst spec to level up as, requiring raid frame and healing addons to do efficiently, and being the most boring in a good group while the most punishing in a bad group. Maybe if you main a healer it doesn't bother you or you don't notice it (you won't have to go through m+ pug after pug with crapo healers because YOU are the healer and control that variable), but I notice it a lot.
When my guild had recruitment open for a tank, we got an avalanche of apps, we even get tank apps when we aren't recruiting one anymore. We have a recruitment currently open for holy pala / disc priest and barely any interest.