That's a good question! I think the answer is momentum. We've been using IRC for a long time. Over the years we've built a lot of internal tools to adopt IRC to our company. IRC is core to our deployment process with pushbot. Certain channels are logged and you can use a webapp to search them. We also maintain an internal fork of the Subway IRC client that integrates with out staff directory and IRC search tools. It acts as a bouncer for all of its users internally, giving them the effect of persistent chat.
"Subway" (found here) is similar, but it is currently under development.
I am not sure if it plans to provide all the functionality of irccloud, but it might be worth a look.