From what I see Swain improved over last year to a point the coaches must have felt they have a guy that can continue to be a better WR and Martin had pretty much shown his ceiling as a player.
If two guys like this are very close and one is younger and shows that 2nd year improvement they look for then you go with the guy fairly even now but having more upside.
I know I heard Swain's name mentioned a lot this year in camp plus he can do some things on ST's Martin cannot do which made it easier.
"The train is leaving the station."