I don't think it was forbidden. The reason I was commissioned to do three DTI novellas was to create the option of collecting them as a mass-market fix-up novel if the opportunity arose. So it was certainly considered to be at least potentially doable. But there didn't turn out to be slots in the schedule for any such reprints.
It appears more likely they chose not to fill available slots with any such reprints, because there have been copious “open slots” over the last couple years.