They possibly found that lots of people using a custom domain were switching away from them, and they wanted to make that harder.

What on earth are they thinking??

No shit??? I started my Medium blog last year and have a custom domain. Why do you suppose they are getting rid of their best feature? Who the hell is going to want to host their blog on someone else's domain??? WTF?

wow i had no idea they did this! this was one of the reasons Basecamp felt able to move over. Now, if you haven't done it already, you can't. It's all medium.com's content.

Damn! Why did they stop?