Endgame's Mr. Hulk was supposed to debut in Infinity War 1 / 7
After his disappearance by the end of Age of Ultron, fans don't see Banner/Hulk for a while 2 / 7
Until he was revealed to be living the gladiator life in Sakaar where Thor found him 3 / 7
After helping Thor defeat Hela, he was badly beaten by Thanos in Infinity War and didn't come out again 4 / 7
He was supposed to Hulk out again in the film said writers Christopher Markus and Stephen McFeely 5 / 7
But his triumphant return didn't fit with the overall dire tone of Infinity War's end 6 / 7
So they decided to save the reveal of Smart Hulk in Endgame instead 7 / 7