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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HMG’s worked, so they were removed! Fusion rifles worked so they were nerfd! Primaries were rarely used so now you have two! PVP is Destiny. PVE never was! Destiny 2 is all the proof anyone needs... Sheep, attack...You know you must...