Think about it, active school shooting. ( god forbid ) deputies get called to the school. You are the first to arrive. You reach in your trunk and pull out a shotgun. The attacker is still moving through the hallways, he has an AK or an AR. Max EFFECTIVE range on that shotgun is going to be maybe 100 yards. Then you can't call the pattern of the pellets. BUT YOUR A$$ as a Sheriff's Deputy is directly responsible for each one!
Now switch to a rifle. ( I don't care which one, pick one ) Now you can meet the attacker FORCE ON FORCE, and hit him, hopefully, from range, before he can hurt anymore people.
That may be the reason, and if that isn't, I can think of plenty more.