Pokemon changing. Why?

Hey guys.

Sorry I am bad at explaining this, I can already imagine all the hardcore Pokemon fans screaming at me.

Over the past few weeks we have been able to catch the best of Pokemon... the region from the first games where Squirtle, Pikachu, Bulbasaur etc could all be caught. I believe it was the Kanto region - We were having a great time, walking out with our little girl and catching as many of these famous Pokemon as we could find. However the game seems to have changed back to a more recent and less familiar region to us where the Pokemon are just not as interesting. Could anyone tell us why this happens and how often the game changes the type of Pokemon you can find? Would it not be better for all region Pokemon to be available all the time?

Thanks and please excuse my lack of knowledge.