It's not a mystery that we'll ever solve, and even the Church refused to take sides with either the Thomists or Molinists.
Predestination follows from the notion that grace and salvation are a free and unmerited gift from God. To say that it's given as a reward for activities of free will is to say that people effectively earn their salvation, and it's Pelagianism. There's nothing that we can do that merits salvation.
It's a delicate balance between sufficient grace and efficacious grace. Whereas a small amount of efficacious grace would suffice for the salvation of one, a tremendous amount of efficacious grace would be required for another, to the point that it would almost have to overwhelm the individual's free will. Only God knows all the criteria that go into the amount of grace He gives to each individual, and we know that He is both all Merciful and all Just ... and that's really all we need to know. St. Augustine actually tried to dissuade people from churning over questions about why some were saved and some were not, asserting that that kind of thinking leads to a vortex of confusion. I don't give too much thought to it, but am content with knowing that God is all Merciful and all Just, and that my pea brain is not in any position to make any judgment regarding God's judgments. How many people lose the faith over such considerations, presuming to sit in judgment of God! Why did God allow this or that tragedy? No Merciful God would allow such a thing. Why did God allow some people to born among animists in the jungle but others were raised by saintly parents and became saints themselves? That's unfair, so now we have to theorize about Baptism of Desire, etc. ... to make is so that God conforms to our judgments regarding would would be fair and what wouldn't. St. Augustine admitted that this thinking was behind Baptism of Desire theory, and he rejected it, saying that "if you wish to remain Catholic," you'll avoid such speculations.
God is perfectly Merciful and perfectly Just ... we know this with the certainty of faith. Our pea brains are not capable of sitting in judgment of Him. End of story.