Why is strategy implementation considered the most difficult stage in the strategic management process?

I think you can answer this question just by thinking
about it practically.  The strategic management process includes planning, implementing
and evaluating.  There are several reasons the implementation stage could be considered
the most difficult:

• Even the best ideas
  on paper do not always turn out the same in practice.
  


• Usually the number of people in the "planning" step is
  only a fraction of the number of people who will be involved in carrying out the plan -
  anytime you increase the number of people - you increase the difficulty of a
  plan.


• People are not robots - even the most meticulous
  (or simple) directions might not be understood by
  everyone.


• Unforeseen obstacles inevitably come up and
  require instant problem-solving -or- going back to the drawing
  board.


• Because strategic management plans are laid out
  with a long-term vision in mind - they do not always work immediately - more often than
  not, they need time to show results.  It is difficult to prove that change is a good
  thing, when the results are long in
  coming.

These are just some of the reasons why
the "implementing" stage is the most difficult.  Planning and evaluating may end up
taking more time - but they are much more straightforward.  The "implementation" is so
action-oriented that it is often impossible to plan for everything that might
happen.