Just a random guess, but maybe it's a regulation thing. Like the empire made such military-grade weapons illegal on civilian ships. And having an illegal military-grade autocannon mounted on the side of the ship would attract attention from authorities.

Of course this is all my own rationalization: it's not like Lucas thought all the intricacies of the Star Wars universe through for their technical accuracy. This is more like literary interpretation, only instead of showing "The Old Man and the Sea" is a metaphor for the tragedy of human existence, we figure out how space ships work!