Yeah...putting on my conspiracy theory hat for a moment:
Gustin has stated that they use a small factory that also produces for Japanese repro companies. From their workshirt product listing copy:
These are likely longstanding relationships - at least older than the relationship with Gustin - and the factory may have obligations to fulfill those customers' orders before they can offer any excess manufacturing capacity to Gustin. No additional capacity --> no workshirt campaigns.
Obviously Gustin doesn't owe us anything here, including an explanation of why workshirts haven't been offered lately. They decide which campaigns to run, and their customers decide which campaigns to back; the relationship is that simple. At the same time, I wouldn't mind hearing from @joshgustin as to whether there are even plans (maybe "intent" is a more appropriate term here) to make more workshirts.