Don't know, but probably there's some arbitrary legal complications associated to personal accounts so their lawyers simply decided to put those accounts into the "enterprise" category to avoid all the messes and now we're seeing collateral damages from this short-sighted decision.
Google apps for your domain is handled by the GSuite team. All their accounts have various extra requirements, like legal checks, not randomly adding features without consulting the admin, having different sets of servers for some services, having different treatment of private data (since the domain admin must be able to have some control over it) etc.
Some product teams within Google don't like all the extra overhead, paperwork, and complexity that adds, so instead they just block all enterprise accounts.