The proximate cause is that Substack was founded by people less likely to censor, who have so far kept control of the company. It may be more meaningful to say that Substack was founded in 2017, and Medium was founded in 2012, and the sort of censorship we're talking about was much more salient in 2017 than in 2012, so Substack was in some sense selected to be more against that sort of censorship.