If you follow any artists or writers or similar on Twitter you'll note that the biggest reason most of them cite for being there is because they have a conversational community that feeds into support for their art.

Substack has a perfectly good mechanism for publishing (paid and unpaid) to support people, but that conversation is missing. It's a pretty obvious move if you are engaged with the overlapping users of Twitter and Substack, and has potential to peel a lot of people out of Twitter if they're primarily there to follow their favorite authors, showrunners, etc.