Like it really is the best remaining avenue for exploring the gemiverse.
The Crystal Gems are all finally relaxing and basically retired on Earth running Little Homeworld.
Steven has gone off to work on himself/discover more of his human half.
Homeworld has basically put the diamonds into a backseat role while it transitions to a democracy, and Spinel is playing jester to the diamonds to keep them placated.
Stuff with towneys is sorted - but Lars and the off colors specifically wanted to head back into space. They have no actual allegiance to Homeworld after everything they experienced and have the most growth potential since they were really just past the start of their own journeys. Lars, after his character growth, honestly makes for a great human everyman to replace Steven in the narrative.