Open_Source_Enjoyer FUTO keyboard uses a license which prohibits a re-distributer of the software to remove the ability to pay to FUTO.
This means that if tomorrow an update releases which requires payment to use then GrapheneOS users would be stuck with the previous version indefinitely.
Might not seem like a big deal to you, and maybe it's an unlikely scenario, but GOS devs obviously don't want to ship software they potentially can't update, and this wouldn't be an issue with any keyboard which uses an actually free license.