Is there a champion design reason Ashe doesn't have an AP ratio on her W?

Talking about it over on [/r/AsheMains]( and it really makes me wonder. I feel like it'd be a super solid boost to AP Ashe/Support Ashe, even if the ratio wasn't crazy. I realize she's an AD champion traditionally, but she wouldn't be the first traditionally AD champion with some AP ratios (she's already got one on her ult) or even the first one with a functional AP build.
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