I keep asking myself why wasn’t I good enough for my biological parents?

Unfortunately, people who conceive children aren’t always equipped to do much beyond that. I survived some pretty colossal neglectful and abusive parenting, and I wasn’t able to make peace with it until I eventually realized that it wasn’t because of anything I did or didn’t do. The deficiencies are on the adult side and not yours. You were a kid, and parents have a basic obligation to provide safety and essentials needed to live and learn. It sounds like you were denied that by people who had no business reproducing, and that is 100% not your fault.