Interesting short post plus comments on David Maister's blog as to the reasons why mergers fail.
From my perspective, a core reason for failure is often lack of clarity about the reasons for the merger in the first place. Even where there is apparent clarity, at least in terms of the reasons being clearly stated, a little digging shows that those involved have not thought the reasons for the merger through.
Just being being clear on the reasons does not ensure success. But without this, success becomes pretty much a matter of luck. And luck is thin on the ground!