As I understand it, the problem for IPv6 is with existing routing hardware and with testing. So if you're going to go through all the effort to add a new octet (which requires changing the IP packet format to allow for the extra space, for example), then you might as well go all the way to IPv6 and solve the problem once and for all.
Not just the IP packet format. Most software ends up storing IPv4 ip addresses in a 32-bit integer (because it fits exactly). All of those pieces of software would need to be updated to work with IPv4's 'new ocetet' as well. As the other poster stated. If everything needs to be upgraded, then might as well do it right the first time.
