I don't know, because he never told us.
I do know it's working for his people though. One not-predestined guy -- a Pharaoh long ago -- was used to show God's glorious release of his people. A few not-predestined guys -- Pontius Pilate, Herod, and the (most of the) Sanhedrin -- did everything needed for Jesus to fulfill his role as Savior.
I also see God fixing my path throughout my life. Had my mother, (who also probably wasn't predestined), not been dying, I wouldn't have called out to the Lord. Had I taken the acceptance into Penn State, I would have been at the wrong place to ever meet my husband. Had I not met him, I probably would have either starved to death, been living on the streets, or both, when I became disabled. Had I not been disabled, he would have had to endure chemotherapy by himself, so he probably would have starved to death, dehydrated to death, or died on the streets.
I love the saying, "Man plans, and God laughs." It fits our lives perfectly.
I really don't know why God doesn't save everyone. I expect he'll explain it when we see him. If he doesn't, we won't remember the question. (Because there will be no sorrow in heaven, and I have trouble with that one knowing it is highly unlikely my mother will be there.)
That is why I believe in free Will. I believe God is love and want every body save, for God so love the world so He invite every body to believe Him because He want every body save. Than is up to ppl to respon yes or no