Because we don't really get enough traffic from there to justify having to maintain WalrusIRC to support multiple networks and deal with the desyncing that occurs often on Omninet. Also, Omninet was originally not my idea. It came into existence after I stepped down from Omni, when they attempted to ditch EFnet completely (but had to keep both due to protests).
I was against switching Omninet a month ago because we still had some users using it, but now we only have 3 left and 2 of them idle on #Cemetech , which is on EFnet. But of course it requires approval of juju and yourself as well.