That is an insightful question.
The likely answer is that before the “fall” of the angels, they were not yet enjoying the Beatific Vision, and so not yet “in Heaven” as we commonly use that phrase. Thus although we often say “the angels fell from heaven” what we really mean is that the angels fell from grace.
In other words, the angelic beings had to undergo a test (just as we do) before they enjoyed the Beatific Vision. Once one “posseses” the Beatific Vision there can be no sin or disordered exercise of the will. But, up until that time the angels were “on probation” (as we are now).
After the testing,the exact nature of which is not revealed to us, some angels rejected God and fell, others remained with Him and merited (not in the strict sense, of course, but rather in fittingness) the Beatific Vision.
Although not participants in the Beatific Vision, the fallen angels also are unable to “change their minds” and repent. Being entirely spiritual their knowledge does not come to them piecemeal like it does for us (i.e through our senses). They therefore have no new data upon which to base a new decision. Furthermore they do not exist, as we do now, in successive moments. They had knowledge, and they chose the outcome.
Of course much of this is seen darkly and indistinctly. But there is some light to be had, given what little we do know.
What do you think?