Yes there was a reason. The evolution of weapons combined with the size of the target and vulnerable areas available the target has. Experience is a great motivator to advance evolution. Emmanuel Gustin has a good page that has tables of MV, ROF, weight of gun, etc. with some other interesting pages like this one. http://users.skynet.be/Emmanuel.Gustin/ ... un-fi.html
It also involves the type of mission said weapon platform is going to be involved in. To take down heavy bombers you needed 20mm or better yet 30mm guns in WW2. HMG just isn't going to be the weapon of choice for that type of mission. The .50 in a battery of 6 or 8 will be effective against piston engine fighters, light bombers and light ground targets. The .30 size rifle guns were basically just a weapon against fighters which is barely of use against a heavier armoured fighter let alone a light bomber.
Large WW2 era bombers and as aircraft became larger with less areas that have a very vulnerable spots is one. Turbines replacing piston engines helped the .50 to be pushed by the wayside. No coolant system, much smaller, less vulnerable fuel system around the turbine, and more resistance of damage to the turbine from HMG rounds, which made the move to explosive cannon rounds mandatory in my mind..
Another is more power to hoist more weight along at higher speeds. A prop fighters of WW2 era could only carry X amount of weight, so you get fewer heavier guns with heavier rounds with less rounds available or more guns with more rounds being able to be carried. Adding in the variables of gun performance also helped in the choice.
Fuel switching from avgas to jet fuel. One doesn't flash fire so good and combat at higher altitudes where both didn't burn little if at all. You need oxygen at the right level for combustion.
I'm sure there's others, but these are at the top of the list i my book.