Hey,
i think because an ownCloud developer isn’t necessarily familiar with Thundberbird plugin development. The ownCloud team probably has more then enough work to do with their own product, starting to develop something they most likely don’t have any knowledge/capabilities of is something i personally (as a user) wouldn’t expect.
It’s a similar topic like discussed in Windows 10 sync client updated and refuses to sync with ownCloud appliance on Drobo 5N, there a NAS vendor has expected the ownCloud developers to provide updates for their NAS system where probably no one of the ownCloud developers is owning such a NAS device.