Originally Posted by Crimson5pheonix
She also has FM as a passive, an AD scaling spread shot, better range, and better AD stats than thresh.
Exactly. Ashe has better base stats for the build/role, her passive functions similarly to Thresh's initial Q proc, and her significantly higher range, on-hit slow, and increased revenue due to her E passive all facilitate her role as an ADC. And that's ignoring the fact that Ashe is one of the least damaging ADC's around and is picked for her utility.
If Thresh is building ADC he can't even use his Q's active without dropping his own damage, as he can't AA while he's pulling someone and he certainly won't want to get in melee range with his target. He can't act as an initiator, which is what much of his kit is about, because he'll be too squishy to get in close range, and his ult just becomes a ridiculously high cooldown escape. Building as an ADC with Thresh is functionally wasting most of your kit, and your Q's passive, strong though it is, isn't nearly enough on it's own to make up for that.