The theory is by risking complete classified documents, one could put others at considerable risk. Whereas the post / guardian would probably be smart enough to redact things that could risk lives.
Any whistle blower who has made the moral choice to make certain information public has a choice around whether to take responsible steps to ensure that the information is released to the public as they intended.
Releasing it only to the media companies is an odd decision, if you want to be sure the information will be made freely available.
Because journalists are known to sit on earth-shattering news. Journalists have the greatest incentives for making the information freely available.