A big part of SAML development was the attempt to satisfy every single requirement put forth by every stakeholder. The outcome of this was that they couldn't come up with a single schema that covered everything (schema and taxonomies that attempt to cover too much have their issues). They couldn't even agree on what to call a user ID. Instead they ended up with a schema to describe schemas, making SAML able to flex to handle nearly any use case, but resulting in a terrible mess and wrecking any sane interoperability.

What we have now are groups like OASIS, the Shibboleth Consortium, eduGAIN, and InCommon, each defining the schema their members agree to support, and an explosion of competing standards.

By the way, even Microsoft can't interoperate with themselves. Traditional ADFS SAML can handle arbitrary multi-value attributes that Azure ADFS rejects.