How can God choose some and not others? Possibly not let's find a totally satisfying answer for our limited minds and tinged with sin. But there are some biblical principles that help a lot:
No one deserves salvation. The questioner may be assuming that God owes salvation to man. But it's not like that. God owes nothing to anyone.
We are all sinners and deserve condemnation. The question is backwards.
We must ask, why does God save some? If God doesn't save everyone, no We can accuse him of being unfair, because he didn't have to save anyone.
And if God, wanting to show His anger and make His power known, endured with much patience the vessels of anger prepared for destruction, and to make the riches of his glory, he showed them to the vessels of mercy that he prepared in advance for glory.
We know that God does everything for his own glory. Therefore, in some way form, His plan of salvation, including election, glorifies Him. In a sense, this is the last word. We don't have to understand how, but we know that glorify, and if so, let's accept it!