After Ho-oh left Ecruteak, numerous trainers came to the Bell Tower.

However, even assuming anyone could call upon Ho-oh...

Without sufficient power, they were trounced by Ho-oh.

That's why the elders made the badges the condition of entering the Bell Tower...

... while obtaining the Rainbow Wing was the condition for climbing the tower.

However, these two are only requirements set by humans.

Legendary Pokemon can observe your soul...because of this, it's said that only those with a righteous heart could even face one.

And you, Hibiki, have brought badge, wing, and heart, and so Ho-oh descended upon the tower, for the first time in 150 years.