I have done some outside catering events used to own a catering trailer. The organisers will be charging thousands of pounds up front to the caters for a pitch. Also all them pitches will not be prime spots some caters will get bad pitches the prefered cater will get the best spots.
You have the caterers travelling cost most of them will have traveled around 100 mile each way burns alot of fuel food as to be bought upfront as well which is a big guess you may have 200 customers or 500. Wages of the staff which are working the event
Also most big events set certain prices ie tea coffee and cans of juice so other caters are not under cutting one another ie 50p for tea or a coffee and a 1£ for a tin of juice etc
Also at the end of the day there has to be a profit involved and some of the caterers would have made cash and some will have gone home bust it is not as easy what people think and most of the price fixing is done by the organisers not the caterers