I have seen suggestions that the rationale for the new trackpad is because Apple needed or wanted it to be thinner for the new MacBook:

This is all presumably to drive a little more thickness of out of the MacBook

or this:

Another casualty in Apple’s ruthless quest for thinness is the trackpad’s clicking mechanism.

Looking at teardown comparisons between the old and new MacBook, it seems that the thicknesses of the two trackpad units are hardly different - the new one may even be thicker! Additionally, the old trackpad looks mechanically simpler - space savings don’t usually ride in on the backs of more large components. I think this is a testbed for a technology that Apple wants to make much more use of in the future — it’s not a space saving objective.