Ashe lacks viability because of her lack of escape when caught out (no mobility) and her lack of an all around steroid ability, if you look at "the main" adcs they all have an ability that amplifies either attack damage or speed or adds an effect to their AAs causing more damage. Vayne has her extra AD from her ult + her true damage, Tristana has her AS boost from her Q, Kog'Maw has his % HP damage & range steroid on his W, Graves has a dash and attack speed boost, Caitlyn has extra damage every so many AAs (not as strong as other steroids which is why she gets out dueled often late), Jinx has her minigun & rockets, Twitch AS on Q and damage + range on R, etc...
The "lower tier" ADCs (Ashe, Urgot) are as such because even though they can bully in lane early, unless they scale fast and hard they will be left behind from their lack of a steroid to their AA damage.
I in no way dislike Ashe, in fact she is one of my better ADCs that I use, I'm just pointing out why shes not played in higher brackets*