Steven, let's delve into this matter together, seeking understanding from God's Word. The passage you're referring to is:
But do not forget this one thing, beloved, that one day is with the Lord as a thousand years, and a thousand years as one day. The Lord is not slow concerning his promise, as some count slowness; but is patient with us, not wishing that any should perish, but that all should come to repentance. 2 Peter 3:8-9 WEB
God's desire is that all should come to repentance, as He takes no pleasure in the death of the wicked (Ezekiel 33:11). However, God also knows that not everyone will accept His free gift of salvation.
so far you understand correctly, but your question was why would God prolong it waiting for everyone to be saved if He knows everyone won't be saved.
Enter by the narrow gate; for the gate is wide and the way is easy, that leads to destruction, and those who enter by it are many. For the gate is narrow and the way is hard, that leads to life, and those who find it are few. Matthew 7:13-14 WEB
So then, it is not of him who wills, nor of him who runs, but of God who shows mercy. Romans 9:16 WEB
God, in His infinite mercy, is patient and longsuffering. He provides every opportunity for people to turn to Him. The fact that He knows not all will repent does not negate His desire for all to be saved. He is not willing that anyone would perish. He is not waiting until everyone is saved. Rather, we should take advantage of His longsuffering and use the time He has given us to share the Good News with others. We should be grateful that He is patient and does not desire that anyone should perish.
Perhaps the answer is that God also has other reasons for waiting and we do not understand them all. We see through a glass dimly, right now. We do not understand, just as you said. But one day we will understand. Let us pray for understanding and patience.
Heavenly Father, we thank You for Your patience and longsuffering. Help us to use the time You've given us to share Your Good News with others. Help us to understand that You desire all to be saved, even though You know not all will accept. Help us to trust You. Give us clarity, open our hearts and minds. Help us to see Your will, Your ways, and Your timing more clearly. Give us Your wisdom, and Your understanding. Help us to accept those things we cannot now understand. In the name of Jesus Christ our Lord and Savior we pray. Amen.
Let us also remember that each day is one day closer to the day God's patience will be complete. Let us be busy about His work while it is still day.