Here's a quote from Emil Pagliarulo from the other thread discussing the Gauss Rifle.
The explanation still doesn't make much sense for the distinction, laser rifles fire laser beams at enemies, while plasma rifles fire bolts of plasma, in both cases, they fit as energy weapons due to using energy based means to cause damage. A Gauss rifle, however, fires a solid projectile, it does use an electrically powered firing mechanism, but the energy is just a means to shoot the bullet, the thing you're shooting is the bullet itself, so it's a projectile weapon. Granted, it isn't exactly a conventional firearm either, so the small guns category does not necessarily fit, but it certainly doesn't make sense for it to be an energy weapon.
There was really no need to use the Gauss rifle if Bethesda wanted more energy weapons to play some roles that are left unfulfilled, after all, as has been noted, the Wattz 2000 seems like it could make a good sniper rifle, and Bethesda could always bring back pulse weapons, or they could even well... you know, do something original? Bethesda put new laser weapons and a new assault rifle and new models of robots in Fallout 3, so why should it be so hard to do a few actually creative energy weapon ideas?