I think there are a few reasons:

• It requires an MMU, so a lot of low-end microcontrollers are excluded.
• Many of its users are in the defense sector. They are likely either unable to contribute to open source or have huge overheads for doing so.
• While the kernel is GPLv2, modifying it is very strongly discouraged, and contributing changes is very difficult because you have either not break or fix the proofs.
• Some of the most exciting potential uses are blocked by the poor support for speculative execution mitigations.