Until UEFI and secure boot, SMM would run code provided by the BIOS. BIOS was updatable, moddable, replaceable. See coreboot and numerous BIOS mods such as wifi whitelist removal.

Trustzone usually runs code from eMMC. These chips are programed in factory with a secret key in the RPMB partiton. It's a one-time operation - the user can't replace it. Without that key you can't update the code Trustzone executes. Only the manufacturer can update it.

Also, any ring -2 code can be used for secure boot locking the device to manufacturer approved OS, enforce DRM, lock hardware upgrades and repairs, spy, call home, install trojans by remote commands, you name it. And you can't audit what it does.