Because IRC-the-protocol currently doesn't have what is requested by the users who prefer Slack over it:

* offline messages * full history access and search * fancy authentication schemes

There are some who try to build a full experience similar to what Slack is proposing; I'm thinking of IRCCloud for instance. But then instead of being stuck inside Slack, you're stuck inside IRCCloud (note: for the nice features, not for the basic messages). What we'd need is an open source IRCCloud server and client, that would then become the new standard... then only can IRC compete.

So, it's not just a software issue, it's a protocol issue. Which is why XMPP was born, and Matrix was born, and all other IM protocols were born. But none of them has reached the point where they can overwhelm all the other ones combined, so in the meantime people converge towards a centralized system because it's easier to be up-to-date.

I believe slack does have a IRC gateway.

A webbased irc client that doesn't suck (and keeps a channel log) would probably be a good solution to making things easy for people unfamiliar with IRC.

Some sort of one click setup for a channel on Freenode or a similar network would probably be pretty popular - especially if you could get some useful bots from the get go (like git ones) .