It is an old Jobsian obsession
The expandability of the Apple II was much of the inspiration for the expandability success of the IBM XT.
Jobs was obsessed by a vision of a single-port serial interface for the Mac. Clearly cut down on the bill for making them.
For the early models, that is indeed all it had (keyboard and mouse connectors aside, the earliest may not even have had another theft from Xerox, 'Appletalk', essentially cut down ethernet, I forget, but nobody much used it in early Cacintosh days).
Serial was much too slow for the time, and it must really have been only about keeping the bill of parts down.
As is this latest move. The USB consortium achieved what St. Steve did not, a series of fast and usable serial interfaces.
Having only one is such a cock-up: wtf, is Apple expecting their cultists to automatically use USB hubs? What point in a laptop if so?