So if railguns are so good, why don't we use them in war?

Despite all of advantages of rail guns, they would be impractical for use in warfare. Railguns require huge amounts of energy fire a projectile at any significant speed. In order to provide these huge amounts of energy we would need an energy source too large and awkward to be carried around on a battlefield.

The enormous amount of electrical current flowing through the rails and projectile can also cause potential problems. The rails and projectile can become extremely hot due to electrical resistance, so hot that the rails and projectile can melt together welding the projectile in place.

As the projectile travels down the rails at high speed sometimes the it can lose contact with the rails and then come back into contact causing arcing and damage to the rails. Railguns suffer so much damage upon firing that a they would only be good for a few shots before they would have to be taken apart and repaired.