All things considered, it was a well written and acted show, but it reached a conclusion this year with the outing of Amy's evil company as a corporate wrong-doer, and Amy publicized in the LA Times as courageous. Tyler got to keep his girlfriend, and they imply that Amy and Levi may be repairing their relationship, and getting back together. So it was a good time to end it.
In fact, with the conclusion of most of the plot threads, I suspect the producers knew, or suspected that the cancellation was coming.