I think i figured it out

Well yes because toys are targeted towards kids.

That was their idea when they started. Cena, Mysterio, Orton, etc. were the popular ones with the kids and they figure the action figures were marketed towards kids. Yet they should also think that once most of the kids have their Cena & Orton they probably won't want another one but instead want everybody else for Cena & Orton to wrestle with. When my friends and I were kids we always wanted who was new in the set not another Hogan or Austin.

But apparently the kids DON'T like playing with Cena figures.

For example, basic 18 appeared at target about a month ago. The Punk/Cara/Hunico flew off the shelves within a couple days. All 3 Cenas (and new camo Cenas, none the less) and a beat up Rhodes remained. Presently, 2 of the 3 Cenas are still there and the third one was purchased by myself, a grown man with no children whatsoever who doesn't even particularly like Cena.

You should win the Nobel Prize

You sir, have a firm grasp of the obvious.

Elementary my dear Watson!

My God it all makes sense now. Lots of kids, kids love Cena, lots of Cena figures.

Thanks you good sir, know I know, and knowing is half the battle.

Well, for a little kid, one Cena is enough. A parent probably won't get anymore unless it's a FlexForce or something that seems different in design. For older kids who use an allowance, why would they waste that money on Cena when they already have a bunch? I can see getting one of each outfit he wears (I did it with Triple H's colored triangle tights in the BCA & TTL era - Jakks made red and silver for TTL and blue and green for BCA), but once you get one, there's no need for others. And Orton is Orton. There's no reason to get more than one of him - his tribal design isn't noticeable like it is with a guy like Cena, only now he has a beard so that's something at least.

They've told us they're aware that the sell rate has dropped for Cena/Rey/Orton, so we'll likely get less of them packed as the year rolls on.