There are two different, though related, reasons. The first is that there's a whole lot more gas in the universe than there is solid material, and thus there is a lot more gas available than solid material in protoplanetary disks. read more
The gas giants of our solar system are Jupiter, Saturn, Uranus and Neptune. These four large planets, also called jovian planets after Jupiter, reside in the outer part of the solar system past the orbits of Mars and the asteroid belt. read more