Slack is successful because IRC web clients are so cumbersome and annoying. Most of the things Slack provides easily could've been provided in a web-based IRC client, but where's the upside for Slack in that?
By adopting an incompatible web model, they lock people and their messages in, and can charge per user in the room. As a good IRC client, the most they could get away with is a subscription charge per client, which has the downside of creating a barrier to entry that limits adoption (requires everyone who wants to use it to pay, v. a central employer or organization that can compel people onto a chat platform).
Users like web-based because it means that they just click a button and the thing they want magically appears with no install process. Companies like web-based because it allows them to box all the secret sauce behind a server that can't be introspected. There's no risk of someone cracking their program and distributing it for free, and they get to keep complete control over the crown jewels, which are now and always have been the data that the program purports to manage.
By adopting an incompatible web model, they lock people and their messages in, and can charge per user in the room. As a good IRC client, the most they could get away with is a subscription charge per client, which has the downside of creating a barrier to entry that limits adoption (requires everyone who wants to use it to pay, v. a central employer or organization that can compel people onto a chat platform).
Users like web-based because it means that they just click a button and the thing they want magically appears with no install process. Companies like web-based because it allows them to box all the secret sauce behind a server that can't be introspected. There's no risk of someone cracking their program and distributing it for free, and they get to keep complete control over the crown jewels, which are now and always have been the data that the program purports to manage.