Bellinger was only called up due to Injuries in LA. They got hit by a ton of injuries and didn't plan on calling up Bellinger till at least mid season, injuries forced their hand, and once he was called up, he got off to such an absurd start they couldn't send him down. Bellingers ability to play both OF and 1B also made him really valuable with all their injuries.
I'm sure Devers will come up soon, but there was a lot of things that happened for Bellinger to be called up that soon. Pederson, Puig, Toles, Gonzalez all went down at same time, with Toles being a season ending injury.
By the way Bellinger was the best fantasy baseball pick up ever. Dude has been a God.