@kanazir Honestly, a lot of it is just time; Kubuntu (and many other distros and flavors) rely on volunteer work, and packaging and testing is definitely a non-zero amount of effort. As well, the latest versions of stuff like Plasma also often depend on newer library versions than LTS releases contain. And finally, there's that the main draw of an LTS is stability anyways! So putting our limited resources towards packaging the latest Plasma for the latest release seems reasonable.

@kanazir At the risk of suggesting a different distro, it's worth mentioning that actually the "latest KDE / Plasma, atop a *buntu LTS" approach is, in fact, what KDE Neon does. So that also exists for folks that want stable Ubuntu LTS guts but cutting-edge KDE goodness atop :)