The way I see it...

At first there were lots of WHM and very few SCH. Most ppl put full INT on Arcanist class due to SMN and most Arcanists now Summoners don't like healing.

At first people considered SCH a "bad healer job" because it didn't have an equivalente to Cure2 or Medica/Cure3 (burst heals).
As a tank, I always loved SCH but some (dumb) ppl thought it was "too weak heals". They do not understand the concept of damage mitigation.


So we had lots of WHM (many bad ones who pretty much only spam Cure2 and run out of MP) and very few SCH early on.
That kinda changed as time passed.
I remember when 2nd coil was new some months ago, EVERYONE was looking for a SCH for a static.
This maybe led some ppl to level up and/or learn to play SCH.

Now it seems we have more SCH than WHM. I also noticed that WHM are very rare nowadays on my server.
Most groups were looking for SCH months ago, now most groups are looking for WHM for Coil and Ramuh.