Most cartoons only last 2-3 seasons nowadays, that's just how TV is. Once they reach that point, they're required to give the voice actors raises. It's cheaper to cancel a cartoon and make a new one. When Avengers: EMH was in development, Disney ordered a full two seasons right away, probably because that's all they were ever going to order (unless it hits Spongebob levels of popularity). That's why shows like Ben 10 change the name of the series every so often -- cheaper to make Ben 10: Alien Force season 1 than Ben 10 season 5.
I like Young Justice and Green Lantern -- YJ a bit more, but it's hard more time to develop, and has really gotten good in season 2.
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quate the ravens never more