I'm a prestige 7 and my top 5 guns are all the pistols. 5-7, Executioner, Tac45, B23R, and the Kap40 respectively. Outside of those it is 3 of the shotguns and I believe it is then the M27.
I use the guns I like despite how good/bad they might be. For instance I have been using the Iron Sight Ballista and love it despite the fact that due to the connection issues it is truly a terrible weapon. I actually can't believe I have almost 900 kills with that thing.
While I agree that the majority of the Prestige Masters I have seen lack weapon variety in their top 5 (usually SMGs with an 870 or FAL in the mix) you can't really blame them. Some people play to win and win exclusively, fun be condemned. The maps in this game typically demand SMGs or CQC weapons which is why you see so many shotguns this time out.
It is extremely easy to do put up at least somewhat decent numbers just spraying from the hip with all but the Chicom (which is only marginally more difficult since it is a burst weapon). Not only that but by the time a person has hit Prestige Master they should know what works for most of the maps.
Just like in BF3 where people have 100 service stars with a specific weapon, they use what works and unfortunately the SMGs pretty much outclass all other weapons in the game thanks to the claustrophobic maps. Yes a person can do well with other weapons but those mostly need attachments to make up for some shortcoming (fire rate, ADS time, accuracy, reload, etc...). The SMGs are all pretty much great with nothing on them.
While I don't blame the higher levels for using what works it is a little discouraging to constantly see the same weapons in killfeeds. I like variety in my CoD and while most of the weapons can compete with each other once attachments are unlocked, who wants to do that when a default naked weapon is just as powerful/ideal for any given situation. Save some of the pick 10 points for other things in your loadout. --- "Cover your receptors Perceptor." Driver picks the music. Shotgun shuts his cakehole.
You can check ANY call of duty player's combat record, and 9 times out of 10 they will have the PDW listed at the top of their weapons list.
The PDW, and SMGs in general, are just good all around weapons. People use them so much because they work for the average play style...don't have to be as sneaky/quick as you would for pistols/shotguns/knives...don't have to be as accurate/tactical as ARs//LMGs/Snipers
Weapon classes other than SMG promote more skill, thinking, and all around different play styles than the average player cares to deal with. These players will settle on the middle of the road, comfortable play style. Just the way it goes.