After so many years, we finally found out the real reason for canceling T.U.F.F. Puppy, and the answer itself came from Ray DeLaurentis, the show's main writer and story editor. "The only reason the show stopped airing was because Nickelodeon was having financial problems and lots of internal infighting with executives. There may have been some political reasons as well. Bottom line… The show performed very very strongly for them and was never really given the credit it should’ve gotten!" - said Ray.
Following from this answer, it can be understood that the show was closed not because of poor ratings, but because of financial problems and the show did not have the attention it deserved from Nickelodeon at that time.
What do you guys think about all this?