Because they had to undo quickly 3 relatively important points of the immediately previous movie:
Ragnarok: Asgard is not a place, it's the people! And we saved the people so we saved Asgard, yay!
Infinity War: Kills half of the Asgardians in the first 10 minutes
Ragnarok: Thor has found a new way to tap into his power, now he's truly the God of Thunder and doesn't even need his hammer!
Infinity War: Purees Thor off-screen in the first 10 minutes. Then makes a really big deal of him needing his hammer back.
Ragnarok: Hah guess what, Loki was not actually dead! And his relationship with Thor is important, look at them eventually getting along!
Infinity War: Kills Loki in the first 10 minutes.
(not sure it was precisely 10 minutes or more)