The animated series director said something kinda funny
I quote:
"In 2004 we learned that we had to change the name of our show from 'Avatar' to 'Avatar the Last Airbender' because James Cameron already had the rights to a movie called Avatar," he wrote (H/T Variety). "Now the sequel is called 'The Way of Water.' If part 3 is called 'The Firebending Masters' we riot."
https://www.gamesradar.com/avatar-the-last-airbender-name-change-james-cameron/
This is pretty funny because Avatar 3 is supposed to introduce a fire tribe.