There is no story here. This was the case of a player with a lot of talent and potential, who was injured a lot during his first three years, and still had too many questions at the end of year 3 to sign an extension based on potential.
It is really rare for players who are not max or near max players to sign extensions after their third year in the league for this reason. It is just hard to agree on a market value on a guy at that point. Generally speaking the player and agent will see a lot of potential for improvement in year 4, and they don't want to leave a lot of money on the table if that happens, but teams rarely agree to pay a premium for "potential" of a player when they still have him under contract for 1 more year, and have his RFA rights for another year.
I don't think it's a given that Bradley is back next year, but I think there is a much better chance he signs an extension this summer after testing the market, than there was that he would have signed an early extension.