Personally i stopped because people stopped making maps for Cube and the SP mode in Cube 2 didn't get much attention :-P. So after a while i had played pretty much everything there was there several times.

(though i haven't checked it for years so i don't know how it is nowadays)

They might have stopped making SP maps not just because the development for that feature was (and still is) weak, but because Quadropolis gradually became barely functional, if I remember correctly. Now it seems that Quadropolis is gone, along with a mod I once wrote to prevent the removal of blood decals. haha

Although I can see why you'd be disappointed at single-player not getting much attention, I think the devs made the right move by focusing mostly on multiplayer which had the largest player base by far. Multiplayer was really my thing for the most part. I just wish it didn't take them something like 9 years to finally ship an update to the game.