IRC is an unusual service in that legitimate use is very low-traffic and easy to handle (Freenode has under 100K users, and human users type at most one message

IRC is an unusual service in that legitimate use is very low-traffic and easy to handle (Freenode has under 100K users, and human users type at most one message per second or so), but it's also attractive for botnets and spam. Running the registration services and anti-spam measures to keep your small network going doesn't, to borrow a phrase, add business value. There's nothing Gentoo gets out of running their own server (besides avoiding once-in-a-lifetime events like this) that they wouldn't get out of using a service where someone is already dealing with all of that.