Suzy's first comment is spot on for the ORIGINAL reason, IMO. XHTML was the predicted future standard because of extensibility, which creates smart documents. Example:
The titanic was indeed a titanic undertaking, eventually they made a movie about it called The Titanic.
A search engine has no clue of the context of the three references to titanic above. But if I do this,
The <ship>titanic</ship> was a <size>titanic</size> undertaking, eventually they made a movie about it called <movie>The Titanic</movie>.
And create my own DTD (Document Type Definition) for this document that pre-defines ship, size, and movie, I can now supply context to the search engines so they know what I mean. Searchers can find more relevant documents in this way.
This concept is fascinating. I thought this would be awesome but guess what? It never really took hold, most likely because it presented far too many opportunities to spam pages.
Returning to the original question, I have the following real-life experiences as to WHY people still use XHTML:
So the software manufacturers are both lazy and lead unaware developers in the wrong direction.
I think those two reasons are why you see XHTML and XHTML markup in what are, in fact, HTML documents.