Gaming can be odd. In space games, it seems logical that the virtual space of the game would be divided into quadrates, and in each of these 'territories' different races would live. We humans have done this to the earth with nation states and within those nations created states, prefectures, and counties. We also do it by putting up fences or walls around our houses in the suburbs.
But this is not true for nature. Swallows migrate regardless of national borders and grasshoppers could give a whit about the fence that separates your grass from mine. Not so though with the Pokemon game. In the game, as well as in Pokemon animation, only certain Pokemon live and can be caught in specific sectors. Why the game was designed like that I'm not sure. Obviously, fish don't live in trees and parrots don't nest with penguins. But nature is quite fuzzy and the Pokemon geography is anything but.
By the way, there has been a new release of the popular Pokemon video game called Pokemon Black and White. It is so popular that Webmasters are postings warnings of slow servers due to all the heavy traffic in response to the release. Spinoff merchandise like Pokemon Black and White stuffed toys (called plushies); Pokemon Black and White cards for the evermore-challenging Pokemon card game, and new Pokemon Black and White promo cards are selling like crazy. The new Pokemon Black and White plush toys inventories are hard to keep in stock as well.
Of course, for the new Pokemon Black and White game a new sector has been created as the place to go to catch the new Pokemon. All the other old Pokemon are still waiting for you trainers as well of course. What is never explained in the game though is why Pokemon do not wander out of their sectors like normal animals would. Are their fences around them keeping them in?
It's just as easy to design a 'random appearance' game as a 'contained' one. I think the game is designed to help the gamer keep sense of what Pokemon he has caught and if wants a particular new one, where to go to catch it. Dividing a game into sectors also ensures that the player will end up seeing all there is to see sooner or later. Like Mario, Pokemon is basically a maze game. You can design this in two ways. One, you can make the game linear like Mario so that each successive level is either more difficult or different in strategy, finishing out at a 'top.' Pokemon is more about winning battles in arenas though in a quest to becoming a Pokemon master. Does the fact that sectors are restricted in Pokemon take away from the game? I don't think so.