So Saturn and Jupiter are made of roughly 90% Hydrogen and helium and are defines as gas giants. Uranus and Neptune have a lot less, about 20%, and havr more heavier elements like oxygen and nitrogen. These are called ice giants. Both are so big and have such high gravity that they have rock/ice cores with strange physics we can only hypothesize about.

The gas giants, after you get past the atmosphere, have such high pressure that the hydrogen becomes like a liquid metal. If you remember your periodic table, elements in a column all have similar properties. The column Hydrogen is in is all metals except hydrogen. The pressures are great enough to force out hydrogen's metal properties. It likely goes straight down to the rock/ice core.

Ice giants dont have enough hydrogen to form a liquid metal hydrogen layer. Instead, after their atmosphere comes a mantle of water, methane, and ammonia ices. These proceed their rock/ice cores.

Now, these are hypothesis based on the planets' composition and properties. Fairly good ones too, though we still have some unknowns.

Now, as to the why this is: the current leading hypothesis is thay during the formation of the solar system, the solar winds pushed more the lighter elements to the outer system while heavier elements were mlre resistant. That left with mostly heavy, rocky material for the inner planets and mostly light, gaseous material for the outer planets.

Hope it helps

Edit: initially wrote "row" instead of "column", fixed