It's really only Starlord and Gamora that gets any real amount of backstory beyond 2-3 sentences too. They get personality beats ("Nothing goes over my head."), but not much backstory.As a counterpoint, that wasn't the character introduction rate.
Yondu was a side character in GG1, It was really just 5 - Starlord, Rocket, Groot, Drax and Gamora. So the second one introduced more.