Some suggestions (from a friend who's a Quins STH) that Marler effectively discharged himself from England camp in order to play. Seems odd that Eddie the authoritarian would allow that, but equally Marler's own views on how he has struggled in the past mentally in camp might mean he's allowed a little more flexibility - especially if he's relatively short on gametime.
Still doesn't quite stack up with Joe/Alfie weren't released. Or perhaps the option was there but Lee didn't take them up on it.