The late German Field Marshal Helmuth von Moltke (1800-1891), reflecting upon the devastation of the Franco-Prussian War, declared that no battle plan ever survived first contact with the enemy [actually paraphrasing here, but the integrity of the quote is secure]. The reason von Moltke is frequently quoted or paraphrased 150 years later is because of the wisdom his comment evokes. And the larger the organization, the more ways a plan can fail.
Planning usually occurs at or near the top of the organizational pyramid. It is executed closer to the bottom of that pyramid. Between top and bottom sit many employees of varying degrees of competency and commitment, any one of whom can miss a crucial deadline, lose a file, execute his or her role improperly, or even deliberately sabotage the plan out of philosophical disagreement or out of anger over being passed over for promotion or denied a raise. The plan could falter because key employees or officials miscalculated budget numbers, or misjudged the market or the competition. A sudden shift in the economy, for example, a major run on the stock market or the exposure of a scandal affecting entire sectors of industry -- Enron, WorldCom, Bernie Madoff, Arthur Anderson, Lehman Brothers, etc. -- could derail a plan, as could the corrupt actions of a single bookkeeper. And, of course, there is always the "Peter Principle," wherein a key employee has risen in the corporation beyond his or her level of competency.
There are many reasons why business plans falter. The main reason is quite simply that humans are fallible, and the more of them are involved in conceptualizing and executing a plan, the greater the risk of systemic failure.