Can you summarize the primary reasons that Lucifer fell and Michael did not?

The point of differentiation between these two beings was the ability to put the self aside in service to others needing assistance, needing a greater flow of love and divine support. This is the essence of the divine perspective—to serve love and to be an agent creating and dispensing love to all, to act in a loving way at all times, and to show love to others, and to the self especially. This can only happen in the absence of ego because if the self is valued above others, this is what leads to a state of addiction in wanting more for the self than might be appropriate to maintain perspective and a balance between serving one’s own needs and the needs of others, so there is a fair and equitable distribution of energies. These two angelic beings were simply different enough that one suffered from needs of the ego more than the other. So Michael has not become corrupted in this way, but Lucifer, in going down that path, found it more and more difficult to stop and then to make the climb back. A fall from grace is a major dilemma that requires a huge effort to correct and he was simply unable to mount the effort in a way to begin to return to his former way of seeing things. This is why it is seen as a fall from grace, because it is quite analogous to falling physically from a great height, and then perhaps being unable to climb the mountain once again to regain the former status. This is largely a difference in the soul makeup. Just as with humans, there are variations among the angelics in the makeup of their souls. They are not clones, they are not identical, they are unique, and so they have differing sensitivities, differing interests, differing characteristics, and affinities, just as with human beings. And this is true throughout the universe, there is great variety in all life forms. They have differing characteristics and behaviors as individuals. It might be quite subtle to a human observer, but the organisms themselves will perceive their uniqueness and the differences of their fellow creatures. So this is the main factor involved here. It is no different than humans, some of whom are quite strong in character, and exhibit extreme loyalty and a steadfast adherence to rules and the needs of the established order, whereas others are free spirits with emotions and intentions that are quite volatile and may have more diverse experiences and demonstration of feelings and attitudes. So some are more stable, and reliable and steadfast than others. This is true among the entire gamut of soul characteristics. While all are divine, there are many colors in the palette of divine expression. This creates diversity of great value for carrying out the extremely wide array of duties the angelics are called upon to perform.