The only reason I can come up with as to why they don't do this is lag, but take this with a grain of salt cause I don't know too much about coding.
I'm sure having Ahri's voice and the normal one up is no problem cause it's only two. But then there comes the problem of up to 4 different voice files playing at the exact same time in addition to all the extra noise in the game(champions, abilities,items, minion deaths, sound effects, visual effects).
And then, if we're being honest, people WILL want more so then you have up to TEN different sound files playing at the exact same time.
I mean sure some people(like myself) disable the announcer because they can get the same info from the side-bar and the over-head text and it decreases a little lag, but even if you have a great gaming computer I think it'd cause some pain.
But again, I can be (and probably am) wrong.