Colin to me this can be explained by -

' we were sorry ' - a state of being, a feeling or an emotion,  not a specific action or event but something that was ' ongoing ', starting before they apologised and continuing for some uncertain time after  -  imparfait

' we apologised ' - a specific event/action, passé composé - the apology was given, done and dusted

Expressing continuing action with the imperfect tense in French (L'Imparfait)