A lot of web standards folk have been online a long time, coming from the era of Usenet, IRC, and listservs. That’s no doubt part of it.
Here’s the thing about mailing lists: they may feel clunky, although I’d argue that’s more due to email clients than the format, but the advantage is that they’re asynchronous. When you have people from all over the world participating, it makes a lot of sense to use listservs. Each message is self-contained, so there’s not the overlapping chaos of synchronous channels like Slack. It’s also a lot easier to isolate topic threads in email, whereas it’s practically impossible in synchronous communication. If you have an email client that will show you how a conversation’s threads branch out, you can be a lot more informed and focused than if you have to do the mental work of untangling a chat log.
Web forums also work, for the same reasons, and I suspect that’s why at least some of the CSS WG work has recently shifted over to GitHub. That allows people to participate in the specific topics that interest them, without having to get all the mail of a listserv. It makes a lot of sense, though there may be unforeseen downsides. I’ll be interested to see if using GitHub holds up over time for them.