Re: Blasts in Boston Marathon
Because the jury for the trial will be drawn from the public. Since that’s going to be a major part of the evidence against these two, they probably don’t want to give the legal defence team an opportunity to say that the jury would be prejudiced by having seen the video months before the trial without the legal defence team being able to try to counter it.
It’s normal in trials to ensure that as much evidence as possible is not revealed until the trial to ensure that jury members do not enter with conclusions already made on the evidence. The defence could use that to argue that a fair trial is impossible and have a mistrial, or to at least have the most serious charges stricken.
In the case of, say, the London bombings and 9/11 bombings, since there was no trial because all the attackers were dead, all video and photos were released to the public.