The problem is, no matter how the numbers are drawn out, what actually happens when the flying bullet meets the air is different everytime. This stuff is all non-linear. There's certainly a 'tendency' of how or which path the bullet trajectory follows, but it always varies more or less - which, under certain circumstances, will determine if the bullet hits or misses.
More bullets in the air means more different factors working at the same time. For instance, if you fire 6 bullets at the same time, in some cases(preferably MANY cases) all of those 6 rounds might hit a certain area within a certain circle of diameter. However, in other instances, some of them might diverge and miss.
The problem is, for non explosive shells, of all those entire set of possibilities, only one ideal instance draws up its maximum potential to do damage. All 6 rounds must converge, and connect at a very small point, in order to do a great deal of damage.
If those 6 rounds are scattered along the surface due to variety of reasons that cannot be figured out by numbers alone(air conditions, wing twist, target plane propwash, vibration, malalignment due to convergence, etc etc..), then the chances of the set of bullets for a single burst doing maximal damage, decreases greatly. A stray bullet might sever a cord or puncture a radiator, but it also might do nothing but make a small hole.
The HE shells of cannons are different. They do their maximal damage all the time. Unless its a total dud, whereever or however the round hits the target it will explode and knock out a chunk from a plane, which will immediately effect its flight capability.
So the problem with multiple machine-gun mounted planes is, that it needs to mount more number of weapons to make up for the lack of pure power, and it still could not be as effective as a cannon round. The USN evaluated the Hispano Mk.II as worth three M2 50cal brownings.
To match a Spitfire mounted with two Hispano cannons a Mustang needs six 50 cal guns, and that still might not be effective if the convergence ranges are wrong, or if the window of opportunity to open fire an a target is too short(since the lack of power means a concentrated length of gunfire is required to immediately damage the target on a critical level).
So, why suffer all that extra weight of six machine guns and loads of ammo, when in overall effectivity only two cannons and its ammo load can offer the same results, or even be superior??