a. Methane did not exist in the solar nebula at the radii of Jupiter and Saturn when the planets formed.
b. Methane reacts with the abundant ammonia clouds in Jupiter and Saturn.
c. Methane does not condense into ice in the warmer atmospheric temperatures of Jupiter and Saturn.
d. The greater gravitational force of Jupiter and Saturn prevents the methane from rising to the upper edges of the atmosphere.
e. The relatively slow rotation of Uranus and Neptune allows methane to migrate to higher levels in the atmosphere and condense into clouds.