EVE peaked at around 40,000 active characters today. Generous guesstimate suggests that about 25-30,000 of those were unique users. Possibly fewer. Tweetfleet's Slack has over 1,000 accounts on it, for comparison. That's not a small percentage when one considers that the majority of EVE players are not engaged with the community beyond just playing the game.
It was originally set up by members of #tweetfleet, EVE's Twitter community, as a better medium for discussions - either in real-time or delayed - that were made difficult by Twitter's 140-character limit. That Slack also doesn't require people to be tied to an EVE client 24/7 is a huge bonus as well. I can lose power at home, or internet, or whatever, then hop back on when things are working again and see exactly what I missed without having to bother people for logs. It also has very good mobile support, and is becoming popular with a number of alliances in EVE for setting up their own personal Slack teams.
There are, at the moment, about 25 CCP devs who are signed up on Slack. I would guess that maybe 10 of them use Slack even semi-regularly, and most of them primarily in the channels specific to the stuff they work on, like #devfleet or #lore rather than places like the default #general channel. They all have better things to be doing than spending time advertising for a service created by players for players. That they choose to use it at all is cool, but not required of them, and advertising it sure isn't their problem either.
Word of mouth is a thing. But so is initiative on the part of the user.
If you can't be bothered to act on things you hear or see, it's not everyone else's problem that you aren't involved.
It is no fucking different from signing up to Backstage.