I think they also wanted to show Themyscara again.
The problem with superhero movie sequels is that they're past the 'origin story', but there's a lot of backstory that need to be explained for someone new jumping into the series.
Actually, I feel that it's an annoying trend to always have an origin story as the first movie. One of the reasons I liked "Spider-Man: Homecoming" was that it rebooted without trying to do the origin story again. Likewise, while Guardians of the Galaxy has a flashback to pre-origin, it starts with Star-Lord already as superhero-y as he's going to get. Origin stories have their place, but characters don't need to start with their origin story. That's not how it is in most of the comics.
I think needing to start with origin stories was mostly because mainstream audiences weren't used to superheroes, so they need an explanation to handle "How could someone possibly be a superhero?" But once one accepts that, then we can just start with characters already as superheroes.
I think the thing the recent Marvel Cinematic Universe got right is that it brought non-comic fans up to speed on the characters though their origin stories. As a comic fan, I'll roll with the idea of an amazon princess who fights crime, but that's a huge buy in for Joe Movie Goer, and needs some explanation so they can get into the concept.
Once the MCU movies had established a world full of super hero stuff, they could relax the origin stories a bit. It helps that Star Lord is (in the first movie) more a Star Wars type character than a superhero character.
Plus, I think the origin story is the strongest story for these characters. It tends to be personal, simpler and a more coherent story. Once you start adding in comic bizzareness like star gods and invisible jets and time travelling versions of existing characters side-by-side with their current versions, things get a little too bizzare for the average moviegoer.
I think the MCU managed to wrangle comic bizzareness towards the end, but I don't think they could have kept it up for much longer. And now that the Infinity War arc is over, I expect the movies to either back way off and kind of soft-reboot, or flounder and finally fall off the pop culture radar.
(Plus, Robert Downey Junior carried that franchise on his back. With him retiring from being Tony Stark, I think the franchise is going to lose an important keystone that kept the edifice standing.)