I haven't played Halo 5 that much but at least in the previous games, running out of ammo means you need to move around the map to collect the dropped ammo from the people you kill to be able to keep controlling the map, which means you give the other team a chance to push back when you go through the weapon loop to replenish, even if it's not as hard a loop as the power weapon ones.
which is why I was irritated by Valhalla BR starts, because map spawn BRs were on top of the hill so you never had to leave the hill and come down and replenish.
unlimited ammo would result in people getting to the best spots and being able to sit back and ping forever, or use saturation fire endlessly (which is what I would do if I had unlimited backpack ammo, it would always be optimal to do suppressive fire constantly). You would have to push up reload times and push down clip size to accomodate unlimited reserve ammo.
