The short answer
Based on the definitions provided, the reason why a wicked problem has no stopping rule is because typically you can apply a stopping rule to a problem because there an acceptable or optimal solution exists which allows you to come to a point where there is no need to continue searching or iterating on a better solution.
The long read
I guess it means that you don't really know whether you are dealing with a wicked problem, but you will know if you are not dealing with a wicked problem because you will have found a solution for it. The problem is that sometimes it is difficult to tell if the lack of a clear solution is because you haven't solved for the problem correctly or if it is just a wicked problem that you are dealing with.
If you feel like the problem that you are solving seems to be a symptom of another problem, and that there might be a catch-22 or some circular relationship it might give you a hint that you are dealing with a wicked problem. Other times you might be just lacking an important piece of information and therefore was not able to factor it into your solution.
A typical wicked problem involves behavioural design involving people where there is resistance or adaptation to change. For example, if you created a solution to a behavioural problem and find that the user group simply evolves their behaviour to create another problem, and you find that each new solution gives rise to a new problem then you are potentially dealing with a wicked problem. However, it could also be that you haven't necessarily addressed the root cause of the behavioural problem, or that it is subject to random or unpredictable forces which means you are unable to work out how to factor it into the solution.
Nevertheless, it doesn't stop people from trying to solve such problems because as I mentioned before, you don't always know when you are dealing with a wicked problem, and it is human nature to keep trying while they think it is worth the time and effort (or sometimes because they have already invested so much time and effort).