I know I said something like this in the Ranger thread, but I'm guessing that it didn't meet the sales that were expected, and it was mainly appealing to a younger audience so I assumed not many Pokemon gamers were that interested in PokePark so I guess that was likely why it never got a third game.
Good point most of the kids today say Pokémon is just another dumb game about monsters fighting monsters we need to act if we want Pokémon to survive when we die we have to get people into it but how I will start a Thread about this in Pokémon General.