Why did the show Captain Planet get cancelled?

The show had degenerated into self parody. Even in the beginning only two of the villains (Looten Plunder and Sly Sludge) could be said to have realistic motives and by the mid point of the series (ie about season 3) even they were going over the top to the point of absurdity.

It is hard to take an environmental show's message seriously when the majority of the villains are either way over the top (like Hoggish Greedly who one episode talks about stripping a forest to make one toothpick), come off as former cell mates of the Joker (Dr. Blight and Zarm), or are too fantastic as well as insane to be believed (Duke Nukem and Verminous Skumm). It become even harder when your two borderline realistic villains also start acting nuts.