Final answer:
Uranus formed closer to Jupiter and Saturn and didn't gather enough hydrogen and helium, resulting in a composition with a higher proportion of ices and rocks. The gas-giant planets in the outer solar system have varying compositions and atmospheres due to their initial formation distances and the availability of nebula material.
Step-by-step explanation:
Uranus appears to have formed closer to Jupiter and Saturn and never became massive enough to trap large amounts of hydrogen & helium from the solar nebula. Uranus and Neptune were less efficient at attracting these gases, resulting in a composition dominated by the icy and rocky building blocks that made up their large cores. Jupiter and Saturn, on the other hand, were able to attract more of the surrounding gas from the solar nebula, giving them a more 'cosmic' composition, rich in hydrogen and helium, similar to that of the Sun.
The likelihood of Uranus and Neptune forming in their current locations is low due to the sparse density of material in the solar nebula at distances beyond Saturn's orbit. Theoretical models suggest these planets were initially closer to the Sun and were later pushed to their present outer positions through gravitational interactions. This information illustrates the complexity of planetary formation and the adjustments in our understanding as new data becomes available.