Without knowing, my take is that it's one of those cases where each state has behaved more like individual countries, rather than a subdivision of a larger nation. So you drivers licens isn't really worth much out of state, at least not for validating your actual identity.
I could be wrong, it seems very unclear what Real ID actually is.
* provide an identity document that's hard to forge. Some states, Hawaii famously, had IDs that were ridiculously easy to forge.
* reduce expiration times so that identifying details aren't decades out of date. Some states (e.g. Arizona) issued IDs with 50+ years of validity.
* require actual verification of identity at issuance
* include SSN details. I have no idea why this was a thing and it's since been removed from the actual federal requirements, but most states require original social security documents to be provided during application.
* to establish residency at a particular address.
If it were just identity and residency, states would accept things like voter registration as sufficient. Yet many states (e.g. California) do not.
Not sure what proof of voter registration would look like here though. Proof of resident address can actually be non-trivial for a lot of even housed people. I probably have some online bills and statements but would have to dig them up.
Some states also don't like accepting online bills. That's the reason I wasn't able to ever get one in CA.
Until someone decided that only people over 21 could drink, and that we needed strong controls over ID. And most people had drivers licenses, so that is what they said they would need for a document