I'm really not sure. But PEK landing fees are among the highest in Asia (and rank pretty far up there worldwide), and I would assume gate fees are similarly high. Which is a disincentive for airlines to use gates and use remote stands instead. So that might be a relevant demand factor in not needing additional gates brought on line.

It's not a matter of bringing more flights into PEK, as ATC already can't handle what they've got (witness many forum comments about standard delays at PEK even in good weather). Also not something like shortage of labor to staff/operate the terminal, since that would be in plentiful supply.

Perhaps, in their zeal to develop and build the "worlds largest airport" the Chinese simply overbuilt, and don't really need the additional gate capacity of 3D. But remember, there are currently plans to develop a second major airport in the south of Beijing. moondog is into airport ops and might show up here with better insights on why 3D remains an empty entity.