Well, when the planets formed 4.5 billion years ago, a disk of hot material (Hydrogen, Helium and tiny bits the other elements) formed a flattened pancake-like gas disk around the then-forming sun. This disk is called the ACCRETION DISK because it was material from this disk that was falling onto the sun... Thats how the sun gathered its mass!

Anyway, there was a strong radial temperature gradient in this disk. Imagine that you can look down onto the disk from the top; you see then ascent sun (protosun) at the center. Then you see this debris pancake of stuff. There was a temperature gradient as you radially go away from the protosun. Temperature was HIGH close to the protosun and temperature was cooler far away. Hence, when the accretion disk of gas cooled, it was easy to get condensation of light compounds where it was cold.

But close to the protosun these elements would remain in the gas state. SO... you can think of a snow line at the orbit of Jupiter... closer in to the sun it was too HOT to make the gas condense... Only the silicates (rocks) and metals (iron) could condense... Hence the Terrestrial planets .... That's why the terrestrial planets ate based on rocks and metals...

Further away, however, where things were cooler, the gases could condense into ice particles. This is why the Jovians are gas rich and the terrestrial are not.