Many reasons:

1) It's irresponsible to dump documents without first understanding them and the consequences of publication.

2) It's 100% contrary to the agreement we made with our source when he came to us and talked about how he wanted us to report on them (if he wanted them all dumped, he wouldn't have needed us: he could have done it himself).

3) It would be impossible for the public to process a huge, indiscriminate dump, and media outlets would not care enough to read through them and report them because they'd have no vested interest in doing so (that's what WikiLeaks learned long ago, which is why they began partnering with media outlets on an exclusive basis for its releases).

4) The debate that we should be having would get overwhelmed by accusations that we were being irresponsible and helping the Terrorists; in other words, it would be strategically dumb to do.

5) There are already lots of risks for people reporting on these documents; there would be seriously heightened risks for anyone involved if they were just indiscriminately dumped.