The guests on events such as LEL are a nightmare. They never turn up on time (headwinds make them late, tailwinds or benign night conditions means they push on to a control that wasn't expecting them). They are tired, fussy, and confused. (They are also very lovely people but not from a caterer's point of view).
Professional caterers usually work to delivering a fixed quantity of meals at a fixed time, and I suspect the disciplines required for that can work against the adaptability needed for LEL.
It's also 'another event' for them, whereas for an amateur, it's as much an experience as riding the event itself and justifies 18 months of planning, timetabling, thinking about the volunteers, etc.
That said, an inexperienced volunteer could easily be overwhelmed by the logistics of feeding 1500 people in a day, which is by far the biggest challenge and one that some of the professional caterers had clearly underestimated.