It's obvious that WordPress being cheaper option in the market attracts similar crowd. And those users may or may not want to pay for stuff or subscribe to paid course. So making Tutorials on WordPress may not end up well for subscription based system. Treehouse did the right thing. If you want to survive in Tutorial streaming business, you have to charge for content which has more demand. So the topics like Java, Rails, Python Django, Data science, ,NET and other web development technologies are more in demand. I am not saying WordPress isn't cutting edge like those topics but it's cheaper to watch YouTube videos for many users. My 2 cents.