Big picture: though we are of different substance, we and Angels are not all that different. We have free will and we have the capacity to love.
We don’t know all the answers, but it is revealed through history that when given a choice, some of the Angels chose not to love.
It’s not that they were made unable to repent, but that their choice was analogous to Human death. After death, we may not repent.
CCC393 It is the irrevocable character of their choice, and not a defect in the infinite divine mercy, that makes the angels’ sin unforgivable. “There is no repentance for the angels after their fall, just as there is no repentance for men after death.”
Read more:
spiritualdirection.com/2015/03/23/can-demons-repent#ixzz3hImvyhwt
After the demons were blessed with the beatific vision, they chose, for whatever reason, death over life. There is no repentance after death, so there is no repentance for demons.