how do we catch legends?

im confused. if theres a full pokedex(up 2 251) why cant we catch any legends or johto pokemon?

The Johto region was never completed. Also Legendaries are only available in League Sanctioned Tournaments to the winner who would of received a prize in which they would have a chance to catch a legendary within the Legendary caves scattered around. In fact, the game was supposed to have up to third generation. And contain four areas within it. Kanto, Orange Islands, Johto, and Sinnoh. This alas, did not come to pass due to it not being completed before the owner had to sail into the sunset of the massive job world.