Big bands were essentially killed [0] by cheaper multitrack recording, synthesizers and samplers in the 80s, when there were no such things as torrent trackers. Even the wonderful although hugely inefficient Napster was more than a decade away. Today artists get most money from gigs, while record labels profit the most from printed music sales and royalties.

[0] The correct term should be "relegated to niche performances". If you love big bands you can indeed listen to them somewhere today, the difference being that mostly for economical reasons they're not anymore the default method used for example to make a film score.