@RIOT why can't Ashe stack her passive with Ruunans?

I just tested her out, is their a reason why she can't stack her passive quicker with ruunans since it does do on hit? Right now she has to hit 5 times even with ruunans to be able to use Q.

and what's the reason that she can Q a tower/inhib again now?

It kinda sucks that she has to hit 5 times before she can use her Q at all meaning she only has one skill to use in the beginning of a fight besides her ult.