He has been working on other projects.
Tyler actually answered your question in a video he posted recently on YouTube, check it out!
In a nutshell, Tyler hasn't been posting Chemistry videos on YouTube because he is working on other projects, one example being the Cellscape VR Bio project.
He also worked on creating and supervising the creation of educational content for the Socratic app--see this blog post for more info on that--so it's safe to say that he has been pretty busy in the past year or so.
That said, I really don't know whether or not Tyler will start making Chem videos again, but he will undoubtedly let everybody know if or when he decides to do so.