the reason that there is so much centralization in OU (and any muliplayer competitive game, really) is opportunity cost. Everyone wants their team to be able to do as much as possible. Using pokemon that can't do as much is a handicap.
Let's say you want a Pokemon with Will-O-Wisp. Let's compare two Pokemon.
Arcanine has
>two good abilities in Intimidate and Flash Fire
>workable offensive stats
>Extremespeed
>reliable recovery in Morning Sun
>Absorbs opponent's WoW
However, Rotom-W has
>Levitate (Ground Immunity)
>Resists Water (esp. Scald)
>Resists Elec, can't be Paralyzed
>Much more and useful Resistances in general with only one uncommon weakness
>Volt Switch can maintain momentum
>Good dual STAB
>Gets Trick
>Immune to (Toxic) Spikes
>Doesn't lose 1/4 health from SR
The amount of things that Rotom-W can do is much longer than the things that Arcanine can do so Rotom-W can fulfill many more roles than Arcanine simultaneously while also fulfilling one specialized role much better (with regards to team synergy). That's why Landorus-T is so common, because in addition to being an Intimidator, it has a Ground immunity without being weak to SR, can pivot with U-Turn, AND it hits hard.
In other cases, like that of Keldeo, it is simply the best at what it does. No other pokemon will hit harder or faster than it without sacrificing one for the other, or with the same useful moves. Talonflame has many flaws, for example, but is undisputedly the best revenge killer in the game simply because Gale Wings makes the foe's speed all but moot.
So it's not necessarily copy-pastitis, anon. It's people realizing "shit, I would have won if Poliwrath was faster, or hit harder" and using BellyJet Azumarill instead so they don't have that problem anymore.
