From what I've heard, we are quite far from the Kanto region--you'd have to travel far by plane or boat in order to reach us. And there are many Pokemon in Unova that are considered to be rare or hard to find in other regions, including Kanto. The same, naturally, is true of Kanto, in that there are many Pokemon there that you'd be hard-pressed to find here in Unova.

Take Pikachu, for example. I have yet to ever see one living within the wild of Unova, and until I met Ash, the only Pikachu I had seen before had been in books or on TV. [Same goes for Venasaur and Snorlax--this is the first time he's ever seen a glimpse of one that wasn't just a description of them in a book or in a movie or something.]