why there's no 3D Touch on XR?

XR has haptic touch and so does 11, 11 pro, 11 pro max. Xs has 3D Touch. You would have to look at the iOS user guide for more information - Welcome - Apple Support

The user guide for iOS 13 is not available yet but should be once iOS 13 is released.

It does not have the hardware to support it. 3D Touch is going away and being replaced by haptic touch on the newer devices according to their tech specs