Because there's hardly any long range maps that cater to you using a mid to long range weapon set. Think of it this way you can cover yourself at mid range with your primary. And at close range you can cover yourself with a shotgun. Now the majority of maps in rotation right now are all pretty much close to mid range maps. Guess what that means? Going with whatever primary you want, having a shotgun as your secondary will cover you for a majority of the engagements you'll face on these maps. The only exception is Shores of Time. Long range engagements are possible here, so equipping a sniper isn't a bad idea on this map. You'll just have to keep yourself distanced mid to long range against other enemy players.