Because he dedicated himself to controlling the power ammo spawns. Its a shitty way to play....but its also very effective. You just memorize the locations and timing of the power ammo spawns....ignore any other game objectives...and then just do laps around the map so your there (usually with power ammo equipped) to control the power ammo when it drops. I hate to have to keep repeating this....but this is NOT an "ammo availability" problem. Its a sandbox BALANCING problem. The primary weapons in this game are---and have always been---UNDERPOWERED. They don't kill quickly enough to be a credible threat to people with OHK weapons and other fast-killing weapons. So instead of balancing their presence....Bungie tries to restrict your access to their ammo....and you have a painful situation when someone (like this) figures out how to bypass that ammo restriction. The solution is not to further restrict ammo. The solution is to BUFF primary weapons so that doing what this guy is doing is less attractive (and successful) as a play style.