Probably repeating someone's thoughts here, but: FreeBSD doesn't need/want to be "popular" as some click-once-to-install desktop OS. If it was, it wouldn't be popular any more for
what it is: A general-purpose OS aimed towards (semi-)professional users who need the full flexibility of some modern *nix system.
That said, there's nothing wrong with distributions or forks building something like that on top of FreeBSD. Well, many of them died, so there
is something wrong: keeping it alive and having enough contributors to do so, but that's more of an organizational problem.
There's also nothing wrong with developing some "DE" specifically for BSD systems, it might actually be kind of nice, but keep it a separate project. Well, for lumina, see above. You also need the manpower to get somewhere.
Trying to do that as part of the FreeBSD project would just draw manpower from much more important things: Maintaining and developing a well-designed (and general-purpose) base system and maintaining ports to build and run 3rd party software on FreeBSD. There's no justification to do so: Only few FreeBSD users would benefit from that work. Some don't even use FreeBSD on a desktop, and from those who do, most are perfectly happy with all the existing window managers and desktop environments available from ports.
Finally, some thought about "eye-candy": Although "good looks" can improve your user experience, and some things (like carefully applied translucency and drop-shadows, NOT over-doing it) can help with quick orientation on a crowded screen, people tend to really over-do it (someone remembers those weird themes for enlightenment wm?). The consequences are worse UX:
- Many things are a matter of taste, over-doing it will lead to designs that most people will just hate
- A lot of "bells and whistles" will draw your attention away from whatever screen content actually matters
So, just be careful with that.