My limited understanding is the short answer is profit. The bolt, locking pieces, trunnion, and barrel need tighter tolerances and are more complicated to machine than the blow back UMP and the short stroke piston systems that HK currently produces. The blow back polymer UMP costs half the money to produce than the welded MP5 but sold for about the same money. The same could be said for the G36 series compared to the HK53, 33K, and 33.
Being a huge fan of the roller locked system, with a Fleming sear and over 20 hosts, I think it is a cool system. But look at the prices. I can buy a Colt AR carbine for under $600. A foreign made roller locked "clone" will be over double that. Even with cheap foreign labor, the machine work with tight tolerances still makes these guns very expensive to produce. And look at the HK SP5K. $2,500 is a lot of money to me.
It is my understanding that no industrialized nation uses the roller locked system in their issue firearms. This is old dead technology. Several years ago Malaysia (a third world country IMHO) sold out of their HK33 rifles and invested in M16s. So if Malaysia doesn't want them who does, other than collectors here in the states? A polymer receiver could be made, as Special Weapons made the SP 10 with roller locked mechanism and polymer receiver. The bolt needs to be reworked such that the extractor spring can't be hyper extended. Why HK never fixed that is beyond me. This is just my opinion.
Scott