Kubuntu has always been an afterthought within Canonical, and it showed - as far as KDE distributions go, it certainly wasn't among the best. Worse yet, despite being the only Ubuntu variant with financial support from Canonical, it wasn't even the best variant; Xubuntu always seemed like a more stable release than Kubuntu, for instance (in all fairness, Kubuntu has seen major strides forward recently).
Now, with the focus on Unity, tablets, mobile, and heck, even televisions, it seems Canonical really hasn't a single reason to invest in Kubuntu. Furthermore, Qt has been added to the default Ubuntu installation as well. And last but not least, as Riddell notes, Kubuntu simply hasn't been a commercial success, and at the end of the day, Canonical is a business.
"This is a rational business decision, Kubuntu has not been a business success after 7 years of trying, and it is unrealistic to expect it to continue to have financial resources put into it," Riddell explains, "I have been trying for the last 7 years to create a distro to show the excellent KDE technology in its best light, and we have a lovely community now built around that vision, but it has not taken over the world commercially and shows no immediate signs of doing so."
And this community is now where it's at, just like with the other Ubuntu variants. Riddell will only be able to work on Kubuntu in his free time from now on, so the project needs to switch to full-on community support. This poses problems, because many tasks are not particularly sexy, and Riddell wonders if the community will be up for it.
I'm not sure if this news really is as bad as it sounds. There are enough other KDE-centric distributions around, and I honestly can't fault Canonical for not wanting to invest in a money pit (especially since there's enough other things to fault Canonical for).