Not surprising that the ratings have fallen. There have been some good ones in the Comedy Central run, but on the average it hasn't been as good as the original series. Better for the show to go out while it still has some quality in it rather than limping along tiredly well after it should've ended, like The Simpsons keeps doing.
The ratings have really been doing no better than South Park. If anything, the reason for the show being axed is because it's a very costly show to back, and let's not forget the actors want royalty for their salaries so all that factors into them canning the show. In contrast, a cheaply produced show like South Park where they can complete like a whole episode in a week and which 95% of the voices are done by the show's creators(even the females) is lot easier to back.
I'm actually excited, since Futurama will be cancelled, that means Aaron Ehasz will probably return to the Avatar verse and write for Legend of Korra.