WHM getting progressively more unnecessary in content?

[Exiled_Tonberry]

So why are most speed runners using SCH/AST instead of WHM/AST when that comp is really that much better?

The only situation where WHM/AST would be useful is in first couple of weeks progression. After that you're going to toss the WHM because there's no reason for the extra safe healing.
And even that's debatable because AST can heal just as well as WHM in diurnal.