It stands for "markup language", and was inherited from SGML, which had includes. Strictly speaking, so did early HTML (since it was just an SGML subset), it's just that browsers didn't bother implementing it, for the most part. So it's not that it didn't evolve, but rather it devolved.

Nor is this something unique to SGML. XML is also a "markup language", yet XInclude is a thing.