IRC is for public channels (and supports direct messages). It’s far from perfect, but I wouldn’t call it terrible; it works very well for its original purpose (and has been for almost 3 decades now).
Servers in a network generally have little to no proper recovery from partitions/splits, and a split may result in changes in moderator/administrative users due to the lack of persistent permissions. There is no attempt to recover discussions during a split, or to apply consistent ordering to communication. Splits, which should be transparent, become a highly impactful event to the users within a channel.
Due to the lack of packetized responses and the simplistic method of determining an active TCP connection, it is possible to (for instance) be disconnected because of asking for a list of channels on a slow connection.
It has a _lot_ of problems, and most of the ones I listed are at the network protocol layer. It is still usable for environments where there are very active administrators and few to no bad actors, but it falls apart quickly on the public internet.
XMPP was primarily about person-to-person communication, with group channels being a separate add-on.
Part of iMessage's popularity is the ability to have group conversations the same way as those person to person communications - to the point where SMS users may get excluded from conversations.
IRC is just a terrible architecture which is exposed in its protocol, but it will survive a long time because you fire it up to participate in a community, not just to talk to an individual person.
IMHO paid tools like Slack got adoption when we had pre-existing protocols and clients for decades prior because it works as a package community-as-a-service (imho, to the point of being a detriment to usability). We had the technology to solve the problem, but the packaging and focus made it a legitimate product.