In order to understand why Europeans ( HK rifles) come in the way they are, whether it is SL8, HK91/92/93 or MR223, we have to know what it is defined as a "firearm" in the Euroland.
First of all, pretty much all of the Europeans firearms ( G36/SG55X/HKG3 ) have the upper received considered as firearms (where they place the SN). There are countries considering the "barrel" as the firearms as well. To be considered a "semi auto only" civilian rifle, the upper receiver must be made semi automatic only and no component could be added to make it go FA. That's why even back in the 80's, HK91 was designed such that the upper receiver cannot mate with a G3 trigger pack.
Back to MR223 and all AR15, the lower receiver is considered the firearm with the SN. This is a new idea to the "Euroland" where they are used to serialized upper. Obviously, in the beginning the Germany government did what they were used to in the past - they consider the upper of the MR223 as the firearm and force the pin position offset so that a component ( a FA lower) cannot be added to make the upper go FA. I guess at the end, HK somehow convinced the german government the "norm" for AR15 type of rifles is to treat the lower as the firearms. Therefore, as long as the lower itself could be not be made go FA with the addition of a component ( the upper), it is good to go and there is no justification for the hole offset.
We also have to understand HK is a big visible arms company in Germany, and the Pacifist movement constantly pushes the German government with anti-war influence and targets HK as an arms manufacturer. There must be some pressure on the German Government to scrutinize HK for everything they do.
In a nut shell, it boils down to what it it is a firearms and what it is a component. HK can put a HK416 upper on a MR223 lower to call it a day (ok, they may still have t o put that stud in the barrel extension to make the german government happy). They are not doing it because their marketing dept has a wrong idea of what the civilian market wants. In the late 90's, their perception of what the civilian market wants would probably be true.
I see it as a marketing department failure, more than anything else. A lot of Europeans simply do not understand the "tactical rifle" culture in North America.