I see a lot of WHMs that are not really good with the class. compared to sch you don't have 25% instant heal 3 times a minute, no back up heal like the fairy incase you are occupied by something else and you also have to watch your mp and enmity levels. quite a few might have switched to sch cause of this or started playing "ez mode" DD.
The thing is either you are made to be a healer or not. and while you have WHMs switching to SCHs, you don't really see SCHs or other classes switching to WHM. and in the end you see less and less WHMs...