Because of the different options that different factions have on their armour. For example, I use scout rifles. Dead orbit, as do FWC and NM, have boots I want that hold more scout rifle ammo. The vanguard ones are only for auto and pulse rifles. So in order to maximize your PVE effectiveness, you have to mix armour from at least 2 different factions. One of them being a pvp faction. Which is stupid and not well thought out. Hope that makes sense.