[AJW]

They could have sold out a baseball game a basketball game or an ice hockey match this whole thing is about spectacle and a one off chance to see something.
The UK supports both a basketball and ice hockey league that are semi professional but no American football league which gives a good indication of the standing of the big four American sports in order of popularity in the UK.
As for American football being on TV its filler cheap foreign programming to take up time between the events that people are willing to pay money to watch Football, Boxing, Golf, Rugby, Tennis, and to a lesser extent cricket.