xhtml seems to me quite a bit simpler to implement than HTML5.

XHTML is hard-structured, and among its negatives, requires being fully downloaded to be parsed and validated. HTML, including H5, has soft-fail modes.

At least that's the justification I generally see. See the Criticism section of the Wikipedia article:

https://en.wikipedia.org/wiki/XHTML#Criticism

That makes HTML5 harder to implement, not simpler.

HTML5 != XHTML. (If that's what you're saying.)

HTML5 breaks soft. It's easier to write, which is what drives content. A parser for which there is no content becomes moot.

(I'd prefer far more rigorous document specification. That's not the Universe I inhabit.)

You're responding to a thread pointing out that browsers are too complicated to implement, which has forced everyone but Google and Mozilla to give up on providing browsers and web standards.

And Google is currently the funding source for Mozilla, giving them a more or less complete monopoly on the future of the web.