Weapon balance has many problems but I think this is the most noticeable.
Combat Rifle and Shotgun both have a dramatic advantage in mag size/fire rate over the Double Barrel and Hunting Rifle, while having nearly as much damage.
They have common ammunition types, and Combat Rifles can be modified up or down to even using .38.
Both can equip suppressors(unlike all of this useless energy weaponry), for stealth and benefit from Mr. Sandman bonus damage.
You can simply buy a powerful Legendary version of either at static vendors, allowing you to not worry much about Legendary loot luck/item scaling for a long while.
They take up little first person screen space relative to the godawful assault rifle.
I've got a character at level 60 that still uses only these two weapons, although I actually have 3 different combat rifles by now just using up the different ammunition types depending on enemies - a wounding .38, double shot .45, .308 with penetrating.
I haven't found any good reason to use anything else aside from aesthetic preferences.