Because timing is everything. The tape was released when it was felt it would do the most damage to its target(s). Keep in mind the actual "real" targets may not be limited to the speakers themselves, they could just be collateral damage in an ongoing battle.
If anybody were to untangle this story, the first questions have to be: Who benefits from this, who is hurt (beyond just the obvious speakers) , and what goal(s) are achieved by the delayed release? If you don't consider the effects of the delayed release, you will never get to the real story. As of right now, it is the biggest clue.
Again, the interests involved here are most likely very powerful, "ends justify the means"-type people. Any enterprising reporter who takes this case on is going to be in extreme danger for sure. That is why it most likely will not happen.