@NexiKuro
Three big inaccuracies here. Firstly, ATLA was not cancelled early. It was not cancelled in any way. It was a critical and commercial success, and a major one at that.
Secondly, Azula's abusive upbringing was very much explored in-depth, in the comics, and how the combination of both her father and her mother's influences (although that was never her mother's intention) warped her mind.
And thirdly, TLoK never had to fear of not getting renewed. All four books of the series were approved and bought up by the network before the first episode even aired, giving massive benefits to the creators in terms of what they could now do with the story, given that the airing was secured no matter what.
In fact, it was because TLoK was bought up in advance by Nickelodeon that the show was not cancelled. TLoK's viewership declined significantly after book one and it continued to plummet all the way to the end of the series. In fact, with the third book the series received such poor ratings that mid-season it was removed from Nickelodeon's broadcast and relegated to online distribution. Under normal circumstances such a series would have likely been cancelled, but since TLoK was already approved and payed for under contract in its entirety, that was not an option.