As a professional software developer, I can assure you that the R&D costs are trivial compared to the QA efforts.
Zwift currently supports 4 major platforms (Mac, PC, iOS, & Android). Each of those platforms has multiple variations (Win 7, 8, 10, IOS 8-12 + ATV, etc).
Adding a new platform is gong to be a major undertaking, especially since Linux has so many variations (distro, hardware, modifications, etc).
Trying to test & support all those variations is going to cost a lot of money.
Even IBM (who, I just read, is buying Redhat) only supports a small subset of Linux distros on some of its enterprise applications (which is the space I work in).