Seem to recall that Nowell picked up an injury over the summer, didn't tour, and May got a bit of an edge on him.
*googles* Ah, there we go. Shoulder nerve injury during last 6N; Exeter put him under the knife to clear up a knee injury while he was going to be out anyway; returned this September and was probably slightly behind the curve to get in during the Autumn.
Personally I thought both him and May has somewhat underrated 6Ns last time around. Their carrying kept putting England on the front foot again and again, despite usually receiving man and ball, without making too many major defensive foul-ups. Their finishing was ropey, May's in particular, but there's more to life as a winger than scoring tries.
I'd like us to persevere with May. I wouldn't quibble too much if he got dropped, if they're seeing him make mistakes on the training field then it might be time to twist, but long term it is better for England to give young players and new combinations time and confidence. Wish it was the Scotland/France game next, as I feel that would limit the gamble either way, but alas next match is everything...