MS literally has to sign and approve the bootloaders from any distribution, or you basically risk your distribution not booting on a majority of x86 systems. And there is always the push by MS to make these bootloaders as restrictive as possible, to prevent the situation where you use one of them to boot some software that will break Windows' FDE. So as a result we end up with e.g. automatic lockdown mode in Linux when booted from a secure boot system.

How did x86 not become more locked down as a consequence of this?

You can disable all of it (on some devices only!) but the war is already lost: most people are not going to do it, so distros have to pass through these hoops.