For example, hypothetically, they may have evidence from spy satellites that show exactly who was driving the SAM TELAR, but they'd like to get his own Facebook posts to reveal same.
That very well could be the reason. Either way it is pretty clear that the Ukrainian rebels shot down MH17 accidentally, thinking that they had shot down a Ukrainian Air Force cargo aircraft.
Of course there will be those that will refuse to believe that no matter how much evidence will be released. Such are the days we live in.