I'm not sure about how others feel, but this is my reason. For a while I mained healer, but I absolutely hate it in Stormblood with the wall to wall pulls that are done. Because of this one big problem, I only play it when I'm leveling that class (usually in PvP or PotD to prevent the insane pulls) or because I need a Cracked Cluster. As of Stormblood, Healer has become the most boring type of class to play while DPS has become the most interesting. Even though I have just started leveling Astrologian (I maxed out White Mage and Scholar first), it is versatile with the mix of shield or regen. The only problem they have is the lack of mana regeneration that White Mages have. However, in a dungeon due to idiot pulls, I just spam heal on the tank 24/7 until they are down to the last 2 or so mobs. It had gotten so bad that one time, I literally fell asleep on my keyboard because I was just standing there pressing heal for so long.

While this might not seem connected to the issue with finding healers for raids in Stormblood, it has completely sucked the fun out of the class for me to the point where I only play it because I need a Cracked Cluster, my Free Company needs a healer for a trial or raid, or I'm grinding for Poetics for an Atma or Anima.

The second issue is gearing up a class. With the tomestone cap and awful savage drops, people are only able to gear up their favorite class. And guess what, when you suck the fun out of a healer class, it's probably not going to be a healer class. Most likely, the people who do play healers have placed their priorities in another class now. That means that either they aren't playing that class for end game or they simply don't care about the content. At my point, I stopped caring about Extreme and Savage content and its something I do when I'm absolutely bored and completely out of everything else to do. I literally finished two Anima weapons (with aetheric condensation) before I even bothered to go do Emanation Extreme.