Mr Light wrote:
I was hoping that Season Two would have Korra only as an airbender on a spiritual quest to unlock her old powers. They should have run with that. Again, we could have seen the Spirit World, and she could try to connect with Aang. So that ending was uplifting and cool but also rushed.
Maybe they would have given the chance, but apparently they'd already completed the writing and voice recording for the first 12 episodes by the time they got approval for another 14. So at the time they wrote episode 12, they didn't know if they'd get to do any more, so they had to give it a decisive ending, not a cliffhanger.