Change. It happens all the time, and sales cycles are no exception. Maybe even more often than in other professions, change happens. You’ve got a mix of personalities and factors. You’ve got a winner-takes-all process with dollars and careers on the line. There is competition, sometimes severe competition. There are others out there whose goal is to screw you over. They make plans, set their tactics to do so in secret and will spring them on you unannounced.
And that’s just on the selling side. On the buying side, you have many of the same factors. Budgets disappear, organizations re-organize, schedules expand or contract, urgency disappears or increases, new solutions to old problems are found, and so on.