The main problem is manpower scarcity. Every time fragmentation increases, typically each project gets a bit weaker - from a situation where they are already very very weak.

> When one project stalls, another can take over.

Desktop environments are too big an effort to see this sort of dynamic. And it shows: the two major projects after 25 years are still the very first two. Around them we have a plethora of little projects who typically die as soon as their main sponsor dies, and contribute very little "R&D" to the main two - in fact, KDE and GNOME more often than not push experiments that the minnows then implement or reject.