For 2 reasons people don't use it. The first being a large amount of people dislike the scope, which you obviously can not change. On a side note, the archetype isn't too great either. Secondly it takes up your exotic slot when that can be used for things like Bad Juju or Red Death. If you have a well rolled hacksaw or an equivalent then that doesn't apply to you too much. I'd also like to add that nearly nobody uses lower aim assist snipers. That's because once you enter the 50ish area it becomes much easier to snipe, but later on into the 60s and 70s it becomes less of noticeable difference, but is still there.