Copyright? Same reason why they can't use the term "avatar"
They use that term 10 times per episode. The original series is also called
Avatar: The Last Airbender and the picture left of your speech bubble is also an avatar and I'm adding all the other meanings that word has to that.
If you are talking about the argument with Cameron's movie Avatar, that mainly had relevance to the life action movie based on the series. Having two completely unrelated movies called 'Avatar' would be quite problematic and confusing, so they had to settle for just
The Last Airbender. There were some issues likewise with having the word Avatar in the title of the LoK series, however that and this are still very different as it focuses on the specific case of having a specific word in the title of a series.
If we would follow your logic, apparently the creators have to keep tabs on all the fan art that is made freely of THEIR series as somehow if they would coincidently use something that looks like an existing fan art, they are in trouble? If they wanted to make an Azula lookalike, they could have done it. Not in the least because they created Azula. You're turning everything upside down.