He acts as He pleases. The new birth is due to the sovereign will of the Spirit. It is with the work of the Spirit we are now concerned, with His work in the new birth, and particularly His sovereign operations in the new birth. The new birth is solely the work of God the Spirit and man has no part or lot in causing it. This from the very nature of the case.
Birth altogether excludes the idea of any effort or work on the part of the one who is born.
Personally we have no more to do with our spiritual birth than we had with our natural birth. No corpse can re-animate itself. The usual answer returned to this question is, Because everybody does not trust in Christ. It is supposed that the Holy Spirit quickens only those who believe. But this is to put the cart before the horse.