Comment by alexandernst

Because whenever somebody asked for:

* 24/7 presence, the reply was "you can do that with a bouncer"

* backlog, the reply was "you can do that with a bouncer"

* full text search, the reply was "that's up to the client implementation"

* file sharing, the reply was "it's already possible with XDCC"

and so on and so forth.

Basically, it's not that Slack is great, it's because IRC as a whole just refused to adapt to what users actually wanted/needed.