I'm assuming cockup rather than conspiracy. However by replacing the ceilidhs with concerts the organisers seem to have misunderstood what their LNE audiences want. They don't just want an opportunity for late-night socialising, they want to dance. Not having a dance floor and replacing the dance bands with concert performers is therefore more than an inconvenience, it completely alters the nature of the event. People are complaining that this is not what they want, and is not what they expected when they bought their tickets or agreed to perform or steward for free. For many of these, the LNEs are their main reason for attending the festival.

They are also complaining that the festival must (or should) have known that this was a possibility much earlier. The information has only now become known, leaving it too late to try to arrange crowdfunding to pay for a dance floor.

If you're suggesting that someone simply forgot to book the dance floor, that is of course possible. However Sidmouth has been running since 1955, you'd expect them to have got the hang of it by now.