I don't know if this is why the people behind secure boot adopted this design, but expiry dates make CRLs smaller, at least in theory. If a revoked certificate expires, you don't need to add it to the CRL because trust is invalidated automatically. By rotating root keys occasionally, you can also keep the revocation list for vulnerable bootloaders smaller, as you can remove any vulnerable bootloader when its signatory key expires.
With the kind of penny-pinching that happens with NVRAM storage on boards like these, being able to reduce the size of CRLs can make a difference.