He was just starting to curry the favor of the general public. He was the subject of a prominent biopic, he received a positive resolution from the UN. He has a lot of supporters, but they're not all fully sold on his philosophy.
Many of his supporters might consider this leak not in the same spirit as the others. The NSA is doing its job, just like the SIGINT agency of any world power. Game theory, tit-for-tat, and mutually assured destruction all make intelligence gathering necessary. As long as one country has SIGINT capabilities, its competitors must match them, even if only for the purpose of defending their sovereignty.
As others have said, the NSA spying on foreign public officials is hardly newsworthy or unexpected. If you believe all SIGINT should cease to exist, then this should also not be news to you, because you already believe these SIGINT capabilities exist, by definition of you opposing their existence. So in no case should this news shock you.
Therefore one has to wonder what Assange's motivation is behind this leak. Based on its timing, it seems far more likely that his motives are personal more than philosophical. It furthers no agenda but his own.
(Or perhaps the NSA is behind the whole thing. This is a pretty nice puff piece for them, and they've gotta be on the offensive finding job applicants these days.)