Just to give the official statement and not just peoples guesses:
Sayfutdinov was level on 8+1 but was suffering with bike issues on the night, whilst Doyle on 8+1 was put out in heat 15 instead, meaning Thompson got the nod. With Rew and Jeppesen suffering from illness and Jepsen Jensen absent, Birmingham put out Hume (3), Vuolas (2+1), Musielak (2) and Edwards (2).
I've been to every home match this season and the crowd level was definitely lower, although remember this was streamed on BSN. People may wish to pay £12 so the family can watch it instead of, for example, x4 tickets at the stadium.
Kings Lynn bank holiday will show if the Witches fans are getting bored or not, but I believe it will be packed as its a day out and local Derby!