There clearly was a discoverability issue with 3D Touch. When you knew which app supported the feature, it was awesome but there was no way of knowing in advance if there was support for it before using it.
Long pressing an icon or anything just to see that nothing happens was just frustrating. Long term effect was that users probably felt this wasn't reliable/supported enough to use the feature at all or at least not in the scale Apple wanted.
The interesting part to me is, replacements for 3D Touch aren't that much more discoverable: you'll need to be randomly long pressing icons, buttons, sometimes specific areas in apps to discover they have additional menus.
Getting rid of 3DTouch feels to me more like a hardware cost reduction (+ harmonizing the system with the iPad) than anything.
edit : fixed typos