John 3:1 ¶ There was a man of the Pharisees, named Nicodemus, a ruler of the Jews:2 The same came to Jesus by night, and said unto him, Rabbi, we know that thou art a teacher come from God: for no man can do these miracles that thou doest, except God be with him.3 Jesus answered and said unto him, Verily, verily, I say unto thee, Except a man be born again, he cannot see the kingdom of God.4 Nicodemus saith unto him, How can a man be born when he is old? can he enter the second time into his mother’s womb, and be born?5 Jesus answered, Verily, verily, I say unto thee, Except a man be born of water and of the Spirit, he cannot enter into the kingdom of God.6 That which is born of the flesh is flesh; and that which is born of the Spirit is spirit.7 Marvel not that I said unto thee, Ye must be born again.8 The wind bloweth where it listeth, and thou hearest the sound thereof, but canst not tell whence it cometh, and whither it goeth: so is every one that is born of the Spirit.9 Nicodemus answered and said unto him, How can these things be?10 Jesus answered and said unto him, Art thou a master of Israel, and knowest not these things?Just a thought. How could Nicodemus be expect to have known about the new birth if it isn't in the OT? That was all the scripture that he had.
God changed His dealings with man again in the NT because even the brightest couldn't see it without the indwelling Spirit. Most saved can't figure out the NT et al WITH the indwelling Spirit. This is why God poured out His Spirit after Christ's Resurrection to aid man to come to Him and to understand Him after they come. It is painfully obvious man was hopeless in understanding God without the Spirit.
You seem well versed in the OT Scriptures, could you have figured it out even now? It makes logical sense that Jesus was demonstrating this point and not the expectation. Nicodemus was a natural man, not a born again, indwelt by the Spirit man.
The natural man can make no sense of the Gospel as clearly laid out and step by step explained in the NT without the Spirit's aid and it is not a hidden mystery as in the OT, it is plain as day and clear in black and white. The few that understood the Lord before the NT were severally aided by God through mighty wonders, signs and miracles and still struggled to keep the faith in between those signs, wonders and miracles. Even throughout Christ's 3.5 year ministry, He still demonstrated signs, wonders and miracles before the Spirit was poured out to aid in man's belief.