Most people who use Slack haven't ever used IRC professionally (or at all) - I've shown IRC to people who love Slack and I usually get "Ugh, it looks so old fashioned and difficult to use" as a reaction. Looks matters a lot if you want people to use it - most IRC clients are old or have designs based on old clients.
Plenty of Slack clones out there that look decent that have similar business models. What would be awesome if there's an IRC server that looked and acted like Slack; nice features like:
- a website to log into like Slack (with benefits like modern web design and Github, Twitter, Facebook login). - an irc client that looks decent on website, desktop and phone clients (and doesn't run on electron). - having multiple channels for one company like Slack channels in a group. IRC currently requires you to register several channels (which is annoying).
------------
As for the difficulty in writing bots for Slack vs IRC, sure it's easier to write stuff for Slack because:
- most Slack bots/services are written by companies who want their third party services to be easily integrated into whatever team communication their client use.
- most custom bots are variation of some bots that someone else has written (anecdotal evidence but pretty sure it's true). I'm pretty sure it's not that different for irc.
- most people don't bother writing bots unless they're willing to play around with bots; difficulty isn't an issue when it comes to writing one and I haven't seen bot development treated as an actual task (it's more of a hack that works and isn't really touched again.)
------------
Honestly, this seems to be a case where the author neglects how real-life people work in favor of code "niceness" (for lack of a better term). There's a reason why most people use Microsoft products (and it isn't because they make great software.)