Or another common question: Why not Freenode? Which sounds to me like "Why are you not on Quakenet? Why not IRCNet? Or EFnet? Undernet?"
There are many IRC networks. #bitlbee picked OFTC as its home network. As did #debian and others. There's no need for a reason. Your IRC client should be perfectly able to keep multiple IRC client connections up so let's not make a fuss about it, okay?