Funny that the L in ldap stands for lightweight, and yet clearly there is interest in a lightweight version of what has become a somewhat bloated protocol.
ASN.1 gets a lot of (imho deserved) crap but it's roughly just a bunch of nested TLV (type, length, value) messages, just smeared with a bunch of legacy and a weird definition language. It's not all that different from e.g. Protocol Buffers. Outside of figuring out what context you're in and thus what message type an integer refers to, there's not much that would be "a hard problem" about it.
That said, the US DoD had a pretty good stab at it, and even today in corners of the defense industrial base you can find companies like Isode that still service that niche. To be fair, x.400 messaging and x.500 directory looked pretty smart back in the day when smtp and passwd were the alternatives. It's just that smtp grew up incredibly fast and quickly outstripped the alternative.