Released for non-current versions of the Wii U firmware. With the Wii U's auto updating feature the amount of developers and users that would have had an exploitable version at the time were a lot less than they are now that an exploit is available up to 5.5.1. Not everyone knew how to block updates or that it was even advised to block updates. Logic would dictate that since the previous exploit was released for non-current versions of the firmware at the time that all new users/developers that would have learned about it would have learned about it after their machine auto updated. Things will come, though just like the release of the current exploit, they might not come as fast as many would like.
The good news is that now that a kernel exploit is released for the current firmware, if you don't want to wait around for someone else to bring homebrew to the Wii U, you have more tools available to develop something yourself - which is way more productive than complaining about it.