Because the AST would then be expected to provide both the large Heals (WHM mode) as well as the barriers (SCH mode).
At the same time.
All the time.
Having it without a restriction would mean the AST has to be balanced around its ability to provide both at once.
It would mean neither aspect of the AST would feel particularly powerful.
Amusingly, having the swap mechanic unusable in combat actually plays to the lore of the AST anyway, since the AST would simply read the future to determine which stance they need to be in for any given fight :P
I would not however, be against some form of really long cooldown that lets you switch modes in combat.