The writings of Jakob Lorber explain why. By the way, Lorber's writings have been described by the famous economist E.F Schumacher as "such a plethora of high wisdom and insight that it would difficult to find anything more impressive in the whole world of literature".
Anyway below is an extract from the instructions Jesus gave when directing his disciplines that show why miracles could never be used by them (source: The Great Gospel of John 131), the short answer is: For the sake of those they are teaching. And if you think about it, it is obvious. If any of Jesus's disciples were to use force or miracles to save themselves or, even worse, to punish those responsible for violence against them, then how could Jesus's teaching of peace ever be freely received in future without fear? Our free will is everything and Jesus's teachings are not be forced on people nor are they to be coerced by any sense of fear or belief in miracles.
Here is the extract:
"This teaching of Mine is thus to be given quite without any external and even less through an inner compulsion to the people and nations in the whole world, and the miracles are only to be performed when the people firstly have a living faith, convincing the whole heart and not plagued by any external doubt and possess otherwise much experience and much knowledge in various things.
No miracles are to be performed before very gullible and superstitious people, because they would rob them of every spark of their already weak free will! And then this new religion of Mine from heaven would be no more useful than their old superstition; for they would begin to give the words from heaven some particular, divinely magical effect, allow it to happen to them and very passively act in all things and places and leave all action according to the religion to a very pious and good man."