We cannot here on earth get an exact answer to the question "why some before others"...but we can rely on the fact that God is perfectly just and good and does everything according to His righteous purposes.
Jer 18:8,10 shows me the basis as to why God has mercy upon some and not others. Just because Calvinism/man made theologies leaves them in the dark on this issue does not leave me in the dark on this issue.
It's impossible for God to lie, Heb 6:18 so God's promise to save the obedient (Heb 5:9) and condemn those that obey not (2 Thess 1:8) are two promises we KNOW with CERTAINTY..
God's revealed word explains those that obey God are the ones that will be saved, Rom 6:16, the one's that work righteousness are the one's accepted with God, Act 10:35. People's man made faith only theology may blind them but does not blind me to this simple bible truth. Certain theologies as Calvinism has God playing the part of the blind man in the dark basement trying to separate black marbles from the white ones. Calvinism is left with a gaping black hole in its theology not being able to explain why some are saved and others are not, even though the bible clearly tells us why.