Here's a quote from Snowden on just that:
"I never published a single document on my own because I believe that the model, the ideal of American government is actually quite a shrewd one,” he said. “I tried to emulate the model of checks and balance. Instead of making a unilateral decision, that ‘The world must know,’ I worked with the free press, institutions that we trust, American journalists.”
Could Chelsea Manning have chosen a better way to release the documents than WikiLeaks? Absolutely. Do I still consider her a whistleblower? Yes - though I would never claim she was as responsible about it as someone like Snowden.