Original: Because Kubuntu is Ubuntu with a KDE desktop, any version available in Ubuntu is also available for Kubuntu.
Simplified: Most of the software used in Kubuntu, is part of the KDE project, but all other software used in Ubuntu can also be used. Because the only difference of Kubuntu and Ubuntu is the default software, you can turn an Ubuntu system into a Kubuntu system, or a Kubuntu system into an Ubuntu system, only by installing some software.