Let me put it this way - you DO know that the Catholic Church teaches positive predestination? That is that some people are predestined by God since the beginning of time to be saved. Even St Paul says so.
We don’t believe in negative predestination though (that is we don’t believe that people similarly are predestined since the beginning of time to be damned, that would go in the face of God desiring, as scripture says, the salvation of all).
If this is the case - that certain people are destined to be saved - why could God not have revealed to Mary, and she to St Simon Stock, that those who faithfully wear her scapular up until the moment of death are among them? Just as He revealed to St Paul that such people exist in the first place?
To be wearing the scapular at point of death means far more than that they’re being rewarded for wearing a piece of cloth. To wear the scapular faithfully means a lifetime, or at least a significant length of time, of devotion both to the Blessed Mother and her Son (to whom she always inevitably leads us). One isn’t drawn to wearing the scapular in the first place without pre-existing devotion to her and Him.
There are requirements attached to the scapular too - regular prayer, chastity according to one’s state in life and so on - and you’re not really wearing the scapular unless you’re fulfilling those requirements too.