If you look around on any DIY web site on the web from first time DIY's, and mainly from first time home owners you will see the words "fast, easy, cheap, do not want to sand, over and over.
What better way to gain that market share then to come up with the so called primer and paint in one.
It would be rare to find any real painter using it unless it was customer supplyed.
It simply does not bond as well.
It's the same as that silly Poly shades Min Wax came up with, first timers think it's great because they think there saving a step. In reality it leaves a blotchy, fisheye looking finish on some surfaces.