Two main reasons I can think of:

Most current BMC platforms are older than seL4

Most run on hardware that is not supported by seL4, or at least on hardware where it has not been validated.

Not to mention that a task manager would be needed as well as tons of other services which aren't provided out of the box, and don't share the verification provided guarantees.

Because people dont want to pay? Openbmc clearly has difficulty getting widespread support as nothing is documented.