Why "Ils ont été"

His brother’s group gave a performance  - an event, a highlight of the afternoon, with clear start and end points - hence passé composé. 

Compare to ‘il y avait beaucoup de choix’ - background information, not a single or specific event; happening at the concert, yes, but no clear time boundary, something that possibly started before they arrived, and may have continued after they left, but is now also finished - imparfait. 

Have a look at the link below - text and the small videos following. 

I think it is excellent for extending beyond theory and principles to further developing a feel for use of imparfait or passé composé, how to ‘choose’ between them and the difference in meaning they convey. 

https://www.frenchtoday.com/blog/french-verb-conjugation/passe-compose-versus-imparfait/

Bonjour Stephen,

"Ils ont été"  expresses a past completed action (passé composé)

"Ils étaient" (L'imparfait) expresses a past continuous action (was, were, in English) or past habitual action (would, in English).

They are not interchangeable because they express different aspects of grammar. 

Bonne continuation

Jim