Why doesn't ash catch any legendary pokemon he sees?
Many of the legendary pokemon serve a purpose like protecting areas
and keeping peace. If Ash were to catch them, that would disrupt the
ecosystem. The show would also be very boring with Ash carrying
around a legendary everywhere. There would be no challenge, and there
would be no reason to watch it. Also, if Ash had a legendary pokemon,
what's to stop Team rocket/plasma/etc from ganging up on him and
stealing it? You really think giovanni is gonna overlook that? Even
if they fail, they're just gonna keep trying, making for an even more
-DT from the Anime FAQ.