battle tower

Kyogre isn't allowed because its a Legendary pokemon, which gives lots of advantages due to its ability as well. The Battle Tower was made for challenges, meaning you're forced to use the trained pokemon, that aren't Legenday, that you have to compete. Honestly, I would never even think of using a uber pokemon like that. :/

Legendaries are to strong to battle with so they don't want you to always win

yeah no legendarys but in daimond/platinum/pearl some guys that are allowed the regis registeel regice regirock and regigas but i dont know if you can use them in sapphire though

kyogre is a legendary. they wont let you use it in battle because they think your cheating... but there are some legendaries that can go inside the battle tower and those are the regis and i think latias and latios but im not so sure

honestly though i think it sucks you cant bring in legendary pokemon in it because my whole team is legendary ones

So far the only ones I've attempted to use Battle Tower and were not able to were Kyogre, Groudon, Rayquaza, Deoxys, Jirachi, and Altaria. I was, however, able to use the three Regi Pokemon, Latios and Latias.

Hope this helps you out.