It's just cheap to run and there are a lot of NES bootlegs floating around the internet, probably more than any other console. The goal of bootleggers is to sell a good looking product with a cheaper than normal price using the cheapest materials possible and the least effort possible to make a sizeable profit so it makes sense to emulate NES games because hardware for emulating them is only a couple cents.
As for why they don't use Atari or Gameboy games I think they use NES games because they have more content than gameboy and atari games. NES games are about as basic you can go while still having a good gaming experience imo.
