Disney owns both franchises.
Well, it was a nice run forAvengers: Endgame. The Marvel Cinematic Universe film that concluded the Infinity Saga was the highest-grossing movie of all time for almost two years, but Avatar is back in the top spot. James Cameron's alien adventure flick, which is just a retooling of Pocahontas, was revolutionary when it dropped. Fans got to experience 3D movies that were immersive and lifelike for the first time in regular theaters.
Disney purchased 20th Century Fox, so Avatar is another one of their franchises now. So, it's really just two Disney titles battling it out for the top spot.
Avatar was able to take back the top spot because of China. The country re-released the film this weekend, giving it the little push it needed to edge past Endgame. However, many fans of the MCU are mad at this switch. They believe Avatar is "gaming" the system by re-releasing the film just to get some extra bucks. However, if the demand is there, why not give the people the supply?
However, one can note that Endgame stayed in theaters for months and months (about a year and a half) just to try and beat Avatar in the first place.