People are free to work on it if they want. If no more peole work on it, it is likely because Cinnamon is unconvincing, you say you are a user of Cinnamon, what prevent you from helping them ?
I tested Cinnamon when it was in the updates-testing, and I was far from being impressed ( but it worked so I gave +1 karma ). There was clearly a lack of polish and small design errorss all over the place ( like non aligned button, non coherent options, or too much useless detail like the 16 different way of minimizing a windows, or the whole plugin applet in the control center that was just empty ), and I found that disturbing.
Then I started to look at the commits, the code is better than Mate ( who is mainly taking code from gnome nowadays ) or mint ( where the python code is not that great ), but there is still some weird stuff going on.
Because that's just a no-op, but starting the python interpreter for nothing. What is fun is that another commit was done to workaround the same problem this file try to correct ( ie having .py not matched by the windows matcher ), on 8374cb9a3661cb3dc71dfd66b3916f5a5adddf5d .
The cleaner solution is to just rename the file, but well...
The whole C part of gnome-shell was forked and is left almost untouched since 1 year. And since they renamed every possible function, there is merge conflict from time to time ( and sometime, they get committed 287f77f771078a5bc2df026dfe2376bf8c469bba ). So it is not hard to see this approach will sooner or later be more costly to maintain, unless they decide to do things cleanly. Because for now, that's not really sitting "cleanly on gnome 3", that's more "duplicating half of gnome-shell".
So there is lots of things to do, and you are right that they could benefit from more coders, but given they forked alacarte, nautilus and said they would fork the rest if needed, I am not sure if they really wish to collaborate and work to a consensus if they can just fork. yet people are free to help, I have seen fedora packagers sending patches so there is some help.
And to finish, as said in another comment, I also have a big screen ( 23" ), I use command line, a browser, evolution on my day job and at home, and I use gnome shell without problem.
Maybe you just didn't found how to use it, for example, the middle click trick ( middle click on the dash run a new instance of the application, quite handy and feel almost natural if you already use the middle click for cut and paste as any old X11 user ).