I regret catching those legendaries. I should had reset for a competitive nature. But why can't I get multiple legendaries, if we can catch multiple Ultra Beasts?
EDIT: there are also different universes right? so shouldn't there be unlimited legendary if there are unlimited universes?
It has always been that way. Legendary Pokemon are exclusive and can only ever be caught once in a single copy of the game. Think of them as mythical Pokemon you encounter in the game (Besides Deoxys which was included in R/S/E and ORAS. Deoxys is still considered a mythical just to add to that).
EDIT: For got to add the Ultra Beast Part. Ultra Beasts are different however . They are not legendary Pokemon (They do have considerable stats for competitive) that's why you can catch more than one. Legends and UB comparisons(all are excluding phione and manaphy): Cannot breed (UB's have this), Usually, if not, have a total base stat of AT LEAST 600 (570 for UB's), and finally, can only be caught once (UB's don't have this). Hope I helped.
Because Game Freak said so.
Edit: also, if people just started playing Pokemon, they would only expect 1 legend per game.