Functionality that has no visual component is invisible to the point that it might as well not exist for 99% of users. Apple has done a lot to teach the built-in functionality like this (e.g. the swipe from top/bottom gestures). But a feature like 3D Touch requires individual apps to support it, and there's a chicken and egg problem here, since it's not worth it for the app developers to spend time on a feature that 99% of their users won't know exists. And because so few apps supported 3D touch, most users never learned to use it (they probably gave up after the first couple apps didn't do anything) and thus there was little pressure on developers to implement it. Neat as it was, I think this feature was doomed from the beginning for this reason.