To me yellow sunlight has never been a good enough explanation for Superman's powers. I think there are lots of factors involved. Heavy gravity, thin atmosphere, evolutive adaptations, genetic manipulation, electromagnetic energy absorption/storing etc. Once Clark is out of Krypton he simply can't be depowered ever again unless he is exposed to Kryptonite. All those weaknesses related to the color of the sunlight are nonsensical. imo. Why Kryptonians didn't explore the universe? Krypton was surrounded by the phantom zone. Everyone who attempted to leave the planet got trapped there for eternity. Jor El discovered a way to traverse the zone and escape the planet "giganormic" gravity pull. The Phantom zone also negated most of the enormous psychokinetic and psionic powers of the Kryptonians while living in Krypton.