I believe it was for PVP purposes and the fact that sometimes even after several minutes has passed since the heavy drop, some people are still running around with machine gun ammo effectively using it as a primary.
The counter to this argument is that in the D2 beta, the fusion rifle gets six shots after picking up power ammo effectively making it the new "machine gun" of D2 so to speak.
Thoughts?
[i]~Nyahh! =^,.,^=[/i]
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You hit you hit the nail on the head, my friend. That is exactly why bungee got rid of machine guns. PVP. To be more specific, PVP sweaty try-hards that were whining about how one player could get as many as 10 kills with a machine gun off of one heavy drop. So now, machine guns are gone. Every single change that bungee has made for Destiny 2 can be explained with PVP. Every single one.