Just wondering... Plus why doesn't it have no guard like honedge and doublade?
Because Game Freak felt they needed to make an ability just for Aegislash. Stance Change makes it a lot more unique because now it gets 2 forms instead of just one. Doublade only has 1 form so they gave it No Guard instead. Another thing that makes Aegislash more unique thanks to Stance Change is King's Shield because it is directly involved with the ability.
Hope it makes sense :D
Probably because Game Freak/ Nintendo wanted Aegislash to have the whole Shield Form Blade Form thing. If that didn't exist, it probably would have No Guard.
Considering that's Aegislash has a shield they wanted an ability to correspond with the appearance of the Pokemon so instead of giving it no guard(it has a shield so their must be a defensive side) they gave it stance change so it can fight and defend.