Because they were the only party competent enough to run a PKI (which is 95% policy) while Linux distros still can't agree on a single boot loader.

shim didn't exist at first. Linux was planning to go without until Red Hat's hand was forced likely because their paying customers demanded it.

It's actually largely because I demanded it, our customers weren't paying attention at all - Fedora was going to be hit much worse than RHEL