This is because of IRCnet is operated by multiple entities and has rather peculiar links, other networks are not like this.

http://dx.fi/alt/ircmap/fdp.png here’s a map of IRCnet, they’re really going out of their way to generate netsplits.

A network operated by a single entity doesn’t need such a silly amount of nodes, you can support hundreds of thousands of IRC uses on a single server.