It's important to look at the user base of who is preferring mailing lists and how it holds up for new users. Mailing lists are the preferred way for most people who've used them for a long time, but new users (newcomers to open source, younger people) definitely prefer to write on a forum and for that experience, it's better if it's a more modern one with relevant features that enhance user experience.
Mailing lists don't do that and you also have zero control over where your emails are kept. Usually, it is very painful or impossible to get anything deleted on a ML, for example.
Regarding IRC, I'll use the same argument I always use. You need a bouncer and you need to be in the same timezone. It's suboptimal for real collaboration and also has no SEO value (logging opens another can of worms regarding privacy and in my personal preference I avoid logged channels wherever I can). With Discourse I have more control, no timezone issues, and several features that I don't have on IRC or a ML, such as polls, nicer image integration, looking up other users, not clogging up my mailbox (doesn't matter if it's filtered into another folder, it's still there), and many more.
That is just my personal opinion, obviously doesn't relate to any of the communities I'm responsible for and developer-heavy communities do tend to need different communication channels than user-focused ones.