The reason for slightly shorter ranges on lasers, as opposed to ballistic or missile weaponry, is that lasers are perfectly accurate and hit instantly. No other weapon in that game does that, though others come close.
Imagine a Super CERLBL with a range of, say, 1200m. This weapon would be pants-on-head retarded; it could blow the limbs off of Mechs and the wings off flyers with ease, pick off BA, and snipe pilots all over the place.
If ranges were increased, damage would have to drop dramatically to keep it remotely balanced.