They were a mounting system superior to rails developed for the XM8, offering significant advantages like lighter weight, impossibility to snag on vegetation or clothes, allowing the sights to be removed and attached without having to re-zero the sight every time, etc. When the XM8 died, the PCAPs died with it.
Some of it has to do it with PCAPs being largely unknown to many people (I didn't know much about them until Kanine told me about them a few days ago), and because H&K owns it, so it's resurrection either by them or another company relies on them.
M16 length barrel in a weapon shorter than an M4. An assault rifle the size of a carbine. Manuvering in enclosed spaces, less awkward length, and other reasons for the M4's adoption are preserved, but you get the barrel length and thus the power of a proper assault rifle.