Why can I not use a certain Instagram username?

[Ecryb]

When I go to change my Instagram username, it says that the username already in use. So, I go and search the username in the Search tab. Of course, the profile with that username does not pop up. I was just wondering why can I not use a certain username. Is it because that username was banned?

[MrPrime]

So people block their profiles from public viewing meaning there might be a user with the name but they might have a private account.

Also if it includes anything sexually orientated or swearing etc they will not be aloud.

[Comicbook Reader]

You can't use it because the name has already been picked. Simply because you can't see the actual Instagram account, doesn't mean it's available. It just means the person who has the name has privacy settings triggered or they haven't actually posted anything. Move on and pick a different name.