Because they will stay in power rather than face accountability (real or imagined) for their actions (legitimate or illegitimate). See Caesar, J and the fall of the Roman Republic.
Framed another way, anyone crazy enough to run for a position in which your opponents will comb through all of your actions with a fine tooth comb to imprison you after the position term is over likely won't plan on their term ever being over.
Who cares? Now we have rulings that say it can't be enforced if it can be convinced that the action of avoiding your term limit can be an "official act" in times of unrest, urgency or doubt.
One must understand that the more safeguards we have to enact retribution in these cases, the better. You're not supposed to point to one after loss of another - you're supposed to point towards as many as possible. Before today, the courts were the one we pointed to the most, and they are no longer nearly as much at our disposal as they were before this morning.