He got dropped after Cech's injury and, aside from this, his contract obviously played a key part in the decision.
Another factor to consider is, unlike an outfield player, there is no real turning back once you drop a keeper. Guys like Aaron and Guendouzi can win their places back. Unless Leno turned out to be a disaster, then dropping Cech would essentially be calling time on his career as Arsenal no.1.
For what it's worth I'm not quite sure I agree with the philosophy, but if it breeds a better mentality within the squad then I see no harm as long as it's just a chance and not a free ticket to the first XI.