Why mailing lists are still quite massively popular in 2015?..
How would you replace them? ( Please don't say Slack. That is just _not_ a good replacement, especially for projects that needs publicly searchable archives. )
IMHO a publicly/semi-publicly logged irc channel would do just as well, but that's even more oldschool.
The alternative is pretty much only online forums. Which imo. doesn't offer a lot of advantages, and most forum software is exceptionally bad (though yes, a few new and nicer ones are coming along)
Discourse NodeBB and Vanilla comes to mind
None of them appropriately support mailing lists, though. Email-based communication is a big deal for devs which contribute on maybe 5+ projects at once and have to manage comms in one central place.
As for Discourse's mockery of a mailing list mode, let's not even talk about it.