User blog comment:Supahbadmarine/By Tzeentch! Here We Go!/@comment-25033299-20130211071701/@comment-2253300-20130211204137

From my understanding the primarchs were never meant to be anything but the generals. I figure the creation of them is a more costly version of what it takes to make a Custodes. Basically I imagine that each of the Priamrchs required a significant investiture of time and resources, thus they were not meant to be mass produced. That is what the Astares are for.