Why does Beijing's new Daxing airport have so few gates?
Airports aren't built with the envisioned capacity in one go. They are built in stages. Just look at Dubai International for example, they're always expanding the concourses, etc.

For Daxing Airport, I found what looks like a master plan (outline for a project's major elements):


Source: kaskus.co.id

It shows more than one (at least two) main terminal buildings, land area for expansion that is serviced by taxiways and aprons, and even an additional runway to the west. There is also plenty of remote stands, which are appealing (cheaper) to low-cost carriers.