Melancon's anxiety issues were well publicized (i remember Jacko warning about his failures in NY, one of the only times he's been right). Aceves doesn't have closer stuff, and having him try to pitch like one proved disastrous. Bard was chasing the money....could've been the answer.
As of right now, they should see what they have in Tazawa. He's a guy that profiles similarly to some other "out-of-nowhere" closers from successful teams like Romo and Motte.