Because the developer wants to, most importantly. And because choice is always a good thing.

So many of these developers are in it out of passion and aren't really being paid. And if part of that passion is working on the other projects that fall within their passions in their spare time. So be it. If they were paid developers and not meeting their goals, that's a whole different story.

Honestly, I would like to see KDE touch and plasma mobile advanced and refined more. But that only means that if I have the skills to help myself I should. Or provide the funds so someone can focus on it. Most of these developers don't owe us anything to get us the other way around.