There are more G3 rifles produced than MP5s, more parts about, MP5s have aways been more desirable, there are less of them, and until recently there were less build options, braces were non existent, SBR paperwork people were either ignorant you could do it or didnt want the aggravation of the paperwork. The concept of building as a pistol also discouraged people. All of it adds up to they command more money and not many people were willing to invest the capital to produce parts for a project that might have their investment sitting on a shelf collecting dust for years. Talk with anyone in the business about having to do a run of 200 of something to bring the cost of something down where it is worthwhile and watch their eyes narrow as they think of their shelves full of those parts years later tying up that investment. Most people in the industry can tell you stories about that.