Probably because Slack has been valued at over $2.5 billion, when it seems (to many people) to be not much more complex than IRC. It is easy to argue that Slack provides value in other ways, but there is definitely a high "hey, I can make that!" factor for many using the tool.
I've thought about this since I first used Slack and decided that they are pretty much (early) Apple (Slack) to Microsoft (IRC), both do broadly the same thing but a tight focus on design (and been fashionable, lets be fair) puts Slack over the top.
That said I still use IRC and will likely do so in the future, lots of open source clients, you can run your own server and you aren't putting your entire history in the hands of a private entity (I'm not the majority).
Also while many of us could create something like Slack the real devil is in the details, getting the UI to the requisite standard, the UX nailed down etc is a lot of work - possibly more than the guts of the clone as you could pull in a bunch of extant systems for that.
