Probably more to do with a shorter over water crossing, and clearing customs
If that were really the first need, then LFAC Calais would be a better choice. Plus, nobody keeps you from passing immigration at either LFAC or LFAT, then fly on to Saint Omer, just a short hop away and with a very enjoyable restaurant. To name just one.
PS observe the terminology: as Brits, you do not need to pass customs. You do need immigration, though, since your !@#$%^&* government wishes to remain outside Schengenland. The terms "customs" and "immigration" are often mixed up, even on aerodrome www pages and even sometimes in the AIP.