The problems, as I see them:
It's not until recently that building a silencer on a form 1 was a cost-effective option for most people. The greatest demand for suppressors has been for mass produced ones from the major manufacturers (AAC, GemTech, Surefire, YHM, etc). The same can't necessarily be said for SBR's and SBS'; which, for the most popular models, can be easily built by replacing a barrel (AR's and 870's are just that easy). if the bulk of SBR's or SBS' are being made on Form 1's, there isn't much of a market for such a service; especially not enough to make it financially viable.
As for DD's, there aren't many licensed DD manufacturers or importers. Other FFL's can legally only do a 'limited' number of DD transfers without getting a DD manufacturer's or importer's FFL. (What constitutes that limited number is, as far as I know, unknown, because I don't believe BATFE has ever actually stated a hard number. Of course, the majority of SOT's who lack DD licenses would be gun shy about handling a DD transfer, as it it's completely foreign ground for most of them, and foreign ground scares a lot of FFL's. So, if there aren't but a relatively small number of licensees who can and will do DD transfers, there's not really a financially viable market for such a service.