Plausibly, entering into relationship with God and abandoning it later could be to God a great evil (analogous to divorce). This is relevant because such a great evil can be such that it would have been better for it to have never existed.1
God can know that starting the relationship (or starting it at time t) would cause the individual harm.1 (For example, precluding a better conversion time, or resulting in more crime-punishment) This is relevant because this would be a needless harm, and avoiding causing them needless harm is the kind of thing a loving and merciful God could plausibly choose to do.
Bringing some non-believers immediately into relationship could preclude a time of conversion for the believer which would yield better results (e.g. a proper longer/everlasting relationship rather than just a temporary one, or an everlastingly higher-quality relationship, or more).1, 2
Bringing some non-believers immediately into relationship with God (marriage1) ones who will leave later, could result in these inviduals being punished more harshly for rejecting (divorcing) God and/or for their sins. This is relevant because being merciful is a property of God's, and mercifully preventing an individual from entering into the divine-human relationship they would not last in would be very merciful. It would save them from God's wrath.
• 2 Corinthians 11:2 -- For I am jealous for you with a godly jealousy; for I betrothed you to one husband, so that to Christ I might present you as a pure virgin.
To a lesser degree the same went for God's relationship to Israel. God frequently likens Israel and Judah to harlots, even going so far as to manifest this by commanding his prophet to marry a harlot.
• Hosea 1:2 -- The Lord said to Hosea, “Go, take to yourself a wife of harlotry and have children of harlotry; for the land commits flagrant harlotry, forsaking the Lord.” … Hosea 3:1 -- Then the Lord said to me, “Go again, love a woman who is loved by her husband, yet an adulteress, even as the Lord loves the sons of Israel, though they turn to other gods
Entering into relationship and abandoning it appears to be worse than never having entered into relationship at all. (And yes, God knows that the sons of Abraham will return to relationship:
• Jeremiah 31:31-34 -- “Behold, days are coming,” declares the Lord, “when I will make a new covenant with the house of Israel and with the house of Judah, not like the covenant which I made with their fathers in the day I took them by the hand to bring them out of the land of Egypt, My covenant which they broke, although I was a husband to them,” declares the Lord. “But this is the covenant which I will make with the house of Israel after those days,” declares the Lord, “I will put My law within them and on their heart I will write it; and I will be their God, and they shall be My people. They will not teach again, each man his neighbor and each man his brother, saying, ‘Know the Lord,’ for they will all know Me, from the least of them to the greatest of them,” declares the Lord, “for I will forgive their iniquity, and their sin I will remember no more.”)