Because the script to Avengers: Infinity War was already underway some time before Taika Waititi’s triumphant threequel was released.
Keeping the God of Thunder with the more serious tone of the previous Thor films would have been an error, and Vulture noted that Chris Hemsworth himself was a huge proponent of keeping this approach to his character in his most recent outings.
Two days of conferencing with Hemsworth and Taika Waititi, and Stephen McFeeley and Christopher Markus had their new approach to Thor in set.
Apparently, the more jocular nature of Thor: Ragnarok’s script was the catalyst for reformatting Thor’s fateful meeting with Star-Lord and his rock star Guardians.