Principally, the WEC’s management team requires confidence that the cost of travelling across the English Channel for a race would be offset, at least in part, via gate receipts.
Which is code for "The BRDC have not agreed to pay us our 'think-of-a-number and double it' hosting fee otherwise they'd be running the event at a loss" - the cost of a channel crossing pales into insignificance compared to sea/air freighting the paddock around the globe.