It is because thunderus and tornadus give 3 attack, not sp attack. So as the leader of the trio, they decided to make landorus, the leader of the trio 3 sp attack.
Here is the link to tornadus and thunderus ev yeilds. See the difference in them then landorus? http://pokemondb.net/pokedex/tornadus AND http://pokemondb.net/pokedex/thundurus