Daisy gave a good answer which aligns with a great insight given by the honorable, deceased Catholic Bishop Fulton Sheen.
In his book "Your Life Is Worth Living", Sheen emphasizes that:
"When an angel decides anything, it sees all of the consequences of its acts with perfect clarity. The principle of contradiction is that a thing cannot be, and be, at one and the same time and under the same formal circumstances. You can never go back on the principle of contradiction. An angel sees the consequences of all its resolutions and choices just as you see that principle. You can never take the principle of contradiction back; it's part of your mental life. When an angel chose to rebel against God, to make itself God, to deny love, it made pardon forever impossible..."
The fallen angels, in light of the "Principle of Contradiction", can never go back to the Goodness they blasphemed. They are beyond space and time, always present in the decision they made.... FOREVER....
Sheen goes on to expound why it is different for humans. "With you and me, it's a little different. We do not always see the effects of our decisions. Our mind is darkened our intellect is weakened, and our will is poor in its resolutions; God allows pardon."
Moreover, as to why the current Angels never fall, it is Traditionally asserted that the Angels, in light of the Glory of God they were bathed in, were all given a one time decision to obey God... Thus, they were confirmed eternally according to their decision either to Eternal Glory or Eternal Damnation.
Angels, like God, eternally live in the Present; unlike humans. Thus,the Principle of Contradiction applied in the perpetual Present sense cannont allow a thing to be and not to be at the same time. Thus, the Angels, prior to eternal confirmation, who chose to disobey God in the Present will always remain in that Present, irrevocable state. This is not true for humans who still have the gift of free will and a future......