I guess it's because 4.012 is the latest Carbonite uploaded to curse...I use Curse to update all other addons except Carbonite and Auctioneer...
In my opinion it would be much nicer to be able to download Carbonite there to, but it seems there's noone updating it regularly there, which is a pity, as updating addons there is as simple as klicking a button and i don't need to fiddle with the Dirs by hand, but I guess there are reasons Faatal and Havok don't use curse for distribution (though I never searched for them
)