Treehouse are skewed towards beginners but I wonder if the market for beginner content is too saturated? Udemy dominates for paid video courses (for beginners) and YouTube covers the free tutorials option.
Also, Treehouse's content is divided into lots of separate videos. It's a big collection of videos but it isn't always obvious how each video relates to another - I wonder if this confuses beginners? I presume this more modular approach was designed to let learners tailor their own learning path. Or to flexibly arrange the content into modular tracks.
Contrast the Udemy approach: The all-in-one course for a language or topic that promises the user a more linear curriculum. Of course, the quality of Udemy courses can vary hugely, but there's no doubt the all-in-one course is appealing to beginner learners.
These is a small but helpful YouTube channel which reviews online learning platforms (Udemy, Pluralsight, etc) called Tech Course Review. Here is an informative review of Treehouse from December 2020:
Treehouse Review 2021: Is Treehouse worth it?