I've always read about spears being all-around superior weapons to swords, from requiring less metal, generally being designed for stabbing which was superior for penetrating armor and more usable in formation, having greater reach, and being very effective against a cavalry charge.
Obviously the Romans knew what a spear was and had used them in battle, prior to the Marian reforms spear wielding infantry made up a significant portion of a Roman legion, and were the more wealthy/experienced troops.
However, the bulk of the army before and after the Marion reforms were equipped with gladii instead. Why would Rome opt to equip it's armies with a short stabbing sword as opposed to short spears like the Triarii used?
I think it comes down to how battles were fought. There are a couple reasons why a gladius would be preferred as the base weapon for the Roman legions.
First, it is capable of both stabbing and slashing, so it is a bit more versatile than the short spears.
Second, reach was actually a disadvantage for the legion. Most battles were fought at extremely close quarters, where the enemy and your friendly soldiers are packed in tightly and your shield is all that separates them. Roman war tactics at the time dictated this type of slow advance, and the one-handed, versatile gladius was the ideal weapon for targeting enemy vulnerable areas around the shield, such as the neck and groin.
http://www.ancientmilitary.com/roman-weapons.htm
It is known that stabbing and slashing were two important functions, and the sight of dismembered bodies ravaged by gladii disturbed Macedonian soldiers opposing the Roman legions (Histories, Book 31, Chapter 34.)
I think it basically comes down to maneuverability and versatility in close quarters. Each legionary was also issued two javelins for longer-distance combat. But trying to wield a spear, or any other longer implement (more than 2.5 feet or so) in a crowd is more dangerous to them and more difficult for you. This website cites Roman historian Vegetius and his description of tactics and formations. Each man in the formation was allotted three feet of space to the sides, and each rank was six feet apart. That's not a lot of room to be wielding a spear. And when the "tortoise" or "wedge" formations were used, that space became even less.
In short, because swords are more versatile and easier to wield in tight spaces.
EDIT: formatting.