what's the reason why Courage the Cowardly Dog got canceled?

No one knows the real reason, some say it had four good seasons and was very popular initially but over time lost its popularity so when the contracts time was up they chose not to renew. Others say the show was too scary for much of its target audiences and was often censored to tone down the horror elements, thus making the network increasingly frustrated and eventually ended the show because of it. The creator of the show has even said its because a pink colored dog doesn't sell well enough in a boys dominated animation marketplace. Whatever the reason, they've been working on a prequel show for a few years and hopefully it will eventually be produced. 

It had a good 4 season run