I've seen double versions of some of those that are more effective than a double artemis. Six Scyla would end your ship so fast you'd forget your name. Echidna might be able to pull that off as well, if a little slower. These guns hit pretty hard and can power through the armor even if they're not effective against it, especially with those kind of numbers. 6 gatling could also be pretty devastating. At that point your unable to keep your armor up let alone your components. 6 Banshee is similar to the other explosive weapons, just with a tad more fire added on.
The only reason most of these don't crop up is because it's easier to toss on two complimenting guns and kill the enemy in the same amount of time (and usually faster) than more guns of a single type.
The other reason they don't pop up in most builds is because they're not as easy to use. 6 mortar is devastating, but you have to get in range, try to get the angle just right and have your gunners hit their mark. If the enemy chooses to move, the slow moving shots have a harder time connecting. I could actually see the gatling gun being pretty effective if you can get in range and have sharp shooters on board.
What makes the artemis capable of being used in these quantities is it's easy of use. You can almost effortlessly line up your shots, disabling your opponent and slowly working them down (of course if you get 6 shooting the same target, it's not quite as slow). Since they're disabled, they're easier to keep hitting.
It's not the power of the gun, it's the ease of it's use. You take away it's trifecta (reduce turning arc) and it's still a good disabling weapon, but it becomes more difficult to use in all situations. It's essentially in the same boat as the field gun was.