Some reasons (don't in order):
- minor hardware support, that means a higher probability of use in obsolete machines with scarce system resources
- small BSD team, so if the team shouldn't commit its own resources on these fat desktop manager, have more time for other things (for example more recent packages on the stable branch)
- makes few sense use similar desktop manager on an openbsd system
If someone has the courage to confess such a use, I will haunt him personally, even in his sleep.
- everyone known the graphics driver bloody mess on 2d and 3d so the decision from
Gnome 3(1) - where the 2d isn't a priority (for a possibility of use even on old system) and so, in the end, you can use similar solution with proprietary driver or, you should hope, that the open specific driver work (possibly without regression from one release to the next release).
Sure on openbsd now is used gnome2 but once the project is dead, efforts to keep it going to increase.
I don't known about kde, if is present the same limitations, because i don't like it, so i don't use that desktop environment.
Naturally i speak only about the desktop manager and don't about the library (gtk and qt) or certain component that don't require too much dependency.
If someone find the lack of a desktop manager however remain Xfce.
Note:
(1) Only on the more recent release something is build for a 2d use.
Naturally drop even mono support.