I'm not sure how they are less usable than Discord. "Everyone" (using quotations here because it's not an absolute thing, but it's almost so) knows how to e-mail, it's one of the most fundamental Internet skills. Using Discord, however, is not, for a large amount of people. Sure, most developers either have had contact with Discord at some point or are capable of figuring it out just fine anyway. But seeing as FLOSS really shouldn't just be about developers (as Drew points out too) and as end users should also be accounted for, e-mail as a basis for coordination and support is a very valid choice.

It's pretty much account-less (in the sense that you don't need to create yet another account), it's easily indexable (there are plenty of web UIs for mailing lists), it's convenient and highly asynchronous, not to mention it's a mature and well established open standard and decentralized protocol, with lots of open tools that fit the spirit of FLOSS in general.

Discord, however, is closed, "unindexable", doesn't work offline at all (with e-mail you can read and compose e-mails totally offline, it's heavier (both in terms of computing resources and data transfer) and full of intrusive pop-ups and whatnot (and has arguably distracting money-seeking features). That's fine and maybe desirable for certain types of communities, specially the instant aspect of it, which is a strong and harmless difference between the two, but it's not fit for the base space for contact between developers, contributors and users.

In my opinion, of course.