They were offered 25 million per episode by Dreamworks to do 3 more episodes and they turned it down. It would have been more money from those 3 episodes than they made on the entire series in the 5 years prior. Clearly they thought ending it was the right thing to do and they were more interested in creating a legacy than in cashing out.
http://www.theverge.com/2013/10/9/48...g-bad-episodes