Multiple factors here.
New jobs are DPS, so there is a huge set of healer mains leveling DPS. Some former healer mains will inevitably become new Samurai or Red Mage mains.
New expansion with a ton of content to get through before hitting EX level. People are working through it at different paces.
And healer jobs being simplified and streamlined is turning some players off, which is another inevitable factor when anything about a job is changed by adding or removing skills, let alone when it happens to all of them.

If healers being the smallest subset of players becomes the new norm, raiders will adjust.