Gotta love the argument of barrel length as an accuracy variable when it's not applicable, given the weapon type, to that characteristic. Think of it this way:

Blaster rifles don't have long barrels for accuracy or velocity, as you'd expect with projectile or magnetically-accelerated weapons. The barrel length is for expanded cooling, so that you can fire the weapon more rapidly. Blasters shoot out some very hot beams, that do a lot of burning to their targets... and in flight. While little baby beam of death is evolving, within the blaster's loving caresses, into the stormtrooper-gutting bolt that issues forth from the weapon, all that heat needs somewhere to go or something to counter it, or the weapon will overheat, with subtle or dramatic results.
Blaster Rifles can fire much more rapidly than pistols, in full auto or burst-firing modes, because the long length of the barrel gives more cooling potential. The faster the weapon can fire, either the more barrel length you notice, or the larger cooling systems on the weapon. Blaster pistols are designed to be fired semi-automatic and as a reliable, accurate weapon when blast quantity is not a priority or not possible; their barrel isn't long enough to handle cooling a rapid succession of shots. This logic applies to standard sidearm pistols, infantry assault blaster rifles, infantry light repeating blasters (which have the longest length to suit the fastest firing), the support rifles, and the medic carbines.

The sniper rifle probably needs the long length of the weapon to handle cooling the larger amount of energy stored in each shot, rather than for sustaining automatic fire. Same with the Medic long-range weapons.

Pistols are marksmen weapons in the Star Wars universe (Read: Han Solo, Wes Janson, etc.) because of their inherent reliability per shot. If you're a good enough shot, you don't need more than one or two bolts fired to down your target or accomplish your objective. Rifles and repeaters are better when you need to manage crowds or lay down a lot of damage in a small (albeit imprecise) area. They're not the standard infantry weapon, because of the relatively small amount of damage dealt in a given time; et tu, not enough firepower. Instead, they're a back-up sidearm (read, not last resort), when you need a few shots to count.

.

.

.

Just my theory.

GC Veteran (0.14-R6) "General Jafar"