It's just done well. It has stood the test of time, and seen nearly 30 years of development in multiple, competing irc server products. A lot of rivalry and different views and attitudes to certain strategies.
IRC is amazing. Servers are linked as leaves to hubs, or as hubs, linking to other hubs or leaves. If the network splits (one server goes down, breaking the tree) it can be quickly relinked to secondary or tertiary servers, restoring connectivity. For its time, and even to this day, it is an impressive protocol that is not only fairly easy to implement from a client perspective, but a very scalable chat server architecture.
IRC is amazing. Servers are linked as leaves to hubs, or as hubs, linking to other hubs or leaves. If the network splits (one server goes down, breaking the tree) it can be quickly relinked to secondary or tertiary servers, restoring connectivity. For its time, and even to this day, it is an impressive protocol that is not only fairly easy to implement from a client perspective, but a very scalable chat server architecture.