Unfortunately, even with sufficient evidence, it takes a lot to convince people of things. A fear of things not being the way they've been accustomed to accepting all their lives, unless they were forced to accept it by other means. And even then, they're quick to associate things that they can't explain away to something that feels better for them to accept than the truth.

I can imagine that the monsters finally being revealed to your planet had an impact, but once the threat was removed, people were quick to find another means to explain it away into something they felt more comfortable with accepting.