Mr Brown,
The plasma weapons used by the Alliance launch pulses of superheated plasma contained inside a containment bubble, correct? And that bubble dissipates with time, causing the plasma pulse to loose its destructive energy and making them less effective at longer ranges?
Lasers loose focus over enough distance, (a laser that is fine tuned to cover the area of a coin at a mile could be a mile across when it reached the moon.)
A sustained stream of Kinetics would spread out too, but they are still individually dangerous (hopefully they are big enough for you to can see them coming and avoid them)
Plasma pulses are a massed cluster of super heated particles over time will loose they're "heat" and then are just massed particles which aren't nearly as dangerous. Its possible that the particles have different properties at different temperatures.
