The major reasons for failure are simple:
Deal fever. Momentum, personal ambition, and public headlines make it difficult to stop a deal once it gets going. Politics and ego make it hard to assess the objective merits of a negotiation once it has started. This applies to boards, too.
Implementation. Those doing the deals rarely implement them. Lawyers send in their invoices, and the strategists move on.
Lack of insight. Nobody tells the truth, and few companies examine where or how the deal makes sense.