It wasn’t quite as straightforward as that. It was a warped idea of revenge for deaths of Muslim children in the US led actions in Syria. So I think being American performer was a key characteristic, as was likelihood of a young audience.
We’ll have to await further announcements from the Austrian authorities about motivation for this attack. IS reaching the stage of imminent attack on large audience of globally famous superstar, and doing so from online radicalisation, is awful. And we know from their history that they don’t limit their targets by sex or to concerts - in Europe they have form for attacks on police/military personnel, Jewish people and synagogues, churches, Shia mosques, museums, tourist resorts, US companies in third countries, metro stations, football supporters, a German music festival, a Christmas market, Westminster Bridge and Parliament, London Bridge shopping areas (Stockholm), railway station (Brussels), random street attacks (France, UK (Streatham))