The answer why Romans preferred the Gladius to a Spear or Pike should be obvious.

Romans were on the offensive almost constantly in their heyday. They took the fight to the enemy, as quickly and devastatingly as possible. They didn't want to poke at their enemies with long sticks....no, as this is clearly more conducive to defensive warfare.

It should be noted that as soon as Rome stopped conquering and started defending, they didn't do as well. This isn't to say this was a primary reason for the fall of the empie, or even overly relevant, but it was clear that the offensive gladius did not lend itself to defending ground.

Also note the increased use of spear auxillaries in the later years of the Empire, when the mindset was defensive, as opposed to the offensive mindset of old.

The Romans no longer desired to bring the fight to the enemy as quickly as possible. No, by this time, they were stationed at arbitrary borders where they had to sit and wait to be attacked.

Sword = offensive

Spear = defensive

Bootsiuv