Imo whm is the healer of duty finder and pugs while sch is the healer of the statics and organised / very functional groups.

Whm can easily heal and compensate bad players due to burst heals while sch can maintain good groups with their shields and sustains.
I guess its just tht since there r a lot of statics and LS farm groups now, a lot of ppl just need a healer who can sustain the party with little effort rather than a healer who can save them from the brink of wiping.