In ice cave I defeated wild articuno I thought I could catch it with master ball it's showed catch Pokemon option I clicked it and I had to fight I defeated the Wild articuno then it said the wild articuno left a frozen shard yeah I got the shard but why articuno cannot be caught?

you cant catch it on the map but you can purchase one in the games corner for 1,250 coins. im pretty sure the only legendary you can catch on the maps would be regice on the map snowpoint town. the other legendaries also cant be caught. moltres drops the blazing shard, zapdos drops the lightning shard, and kyogre drops the water shard. these can also be bought in the games corner: zapdos and moltresfor 1,250 coins and kyogre for 7,500 coins