It sounds like Treehouse is focusing on programming languages. Nothing wrong with that. That may include frameworks (React, Angular, etc.). But even then something you need to keep in mind is WordPress isn't a language (or a framework). It's an application built with the languages and frameworks.

Don't get me wrong, I think there is most definitely value in WordPress education, but if Treehouse has decided to focuse purely on programming language education it makes perfect sense why they would drop WordPress. Different educational focus. Different audience. Easier to focus resources, marketing and messaging from a brand standpoint.

From a technology education standpoint it is more valuable to learn PHP and Javascript the programming languages than to simply focus on WordPress flavor of each. If you know PHP and JavaScript it'll make it easy to learn WordPress.