If God is going to fix our "nature" why didn't He do it from the beginning?

Improving our nature is one purpose of our life on earth. God put us fallen people on Earth and said

Be ye therefore perfect (Mathew 5:48)

God won't make us be perfect because he wants us to choose. That's why he gave us agency (or free will). The world provides opposition God. We can choose to be Christlike because worldly influences entice us to be unchristlike. But Christ has promised

To him that overcometh will I grant to sit with me in my throne" (Revelation 3:21)

It is these people, not necessarily everyone, who will have a new nature. To further clarify, these people at least have a changed nature such that they are "born of God" and believe in Jesus, as declared by John:

4 For whatsoever is born of God overcometh the world: and this is the victory that overcometh the world, even our faith.

5 Who is he that overcometh the world, but he that believeth that Jesus is the Son of God? (1 John 5:4-5)

If the old order of things is going to pass and God is going to make everything new (Human Nature) then why didn't he do this from the beginning?

First, Revelation 21;3-5 says nothing about (Human Nature). From the beginning;

Genesis 1;26

And God said, let us make man in our image, after our likeness: and let them have dominion over the fish of the sea, and over the fowl of the air, and over the cattle, and over all the earth, and over every creeping thing that creepeth upon the earth.

Genesis 2;7,23

And the Lord God formed man of the dust of the ground, and breathed into his nostrils the breath of life; and man became a living soul. 23 And Adam said, This is now bone of my bones, and flesh of my flesh: she shall be called Woman, because she was taken out of Man.

Genesis 1;31 And God saw every thing that he had made, and behold, it was very good. And the evening and the morning were the sixth day.

Genesis 2;16,17

And the Lord God commanded the man saying, of every tree of the garden thou mayest freely eat: 17 But of the tree of the knowledge of good and evil, thou shalt not eat of it: for in the day that thou eatest thereof thou shalt surely die.

Genesis 3;12,23

And the man said, The woman whom thou gavest to be with me, she gave me of the tree and I did eat. 23 Therefore, the Lord God sent him forth from the garden of Eden, to till the ground from whence he was taken.

I John 3;4 Whosoever committeth sin transgresseth also the law: for sin is transgression of the law.

Romans 5;12 Wherefore, as by one man sin entered the world, and death by sin; and so death passed upon all men, for that all have sinned:

Psalms 51;5 Behold, I was shapen in iniquity,and in sin did my mother conceive me.

Because of Adam's fall, we are sinners by default. Adam was created, not born of the water and of the Spirit. We remain sinners unless we are born of water and of the Spirit.

John 3;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.

I John 3;9

Whosoever is born of God doth commit sin; for his seed remaineth in him: and he cannot sin, because he is born of God.

II Corinthians 5;17

Therefore if any man be in Christ, he is a new creature: old things are passed away; behold, all things are become new.

We are made new here on earth, we cannot enter the kingdom of God in a sinful condition.

Revelation 22;14

Blessed are they that do his commandments, that they may have right to the tree of life, and may enter in through the gates into the city.

What is to stop us from rebelling against God as Adam and Eve did?

Nahum 1;9 What do ye imagine against the Lord? he will make an utter end; affliction shall not rise up a second time.

Revelation 20;10,14

And the devil that deceived them was cast into the lake of fire and brimstone, where the beast and the false prophet are, and shall be tormented day and night for ever and ever. 14 And death and hell were cast into the lake of fire. This is the second death.

Revelation 21;8

But the fearful, and unbelieving, and the abominable, and murderers, and who*emongers, and sorcerers, and idolaters, and all liars, shall have their part in the lake which burneth with fire and brimstone: which is the second death.

The devil and the wicked are destroyed, they cease to exist. The new creatures in Christ, cannot sin.