Because the '73-'76 k frames and motor mounts changed to a different style.
The '67-'72 were a block/rectangular style mount while the '73-'76 are a cylindrical style mount; not interchangeable.
The above mount kit is much easier that going through the hassle of changing
the k frame and having to take much of the front suspension loose and removal of the steering box. Save yourself a lot of time!