According to Calvinists, why does God not predestine all to Heaven?

God did predestine or designate some before the disruption of world to be holy and flawless in His sight for the the place of a son for Him Through Christ Jesus; In accord with the delight of His will. Eph:1

Being a son, ties in with His purpose in having an administration of the complement of the eras, to head up the universe in the Christ in the heavens as well as on the earth. Eph.1:10

Paul prays that the eyes of ones heart to be enlightened To perceive what is the hope of His calling. Eph. 1:18.

Which is his body , the complement which is His fullness of Him who fills all in all. Eph1:23

Those who are called will be a part of bringing the whole creation into Christ and under his headship.

God IS the savior of All men, especially those that believe.

for for this we both labour and are reproached, because we hope on the living God, who is Saviour of all men -- especially of those believing. 1.Tim: 4:10

Those especially that believe are those who are chosen in Christ before time, given the faith to believe what Christ has done and will be part of bringing the whole creation out of corruption into the glorious freedom of the children of God.

Christ Jesus came in the world to save sinners and in the end, all will be praising Him because He did exactly what the Father had destined Him to do! 2 Tim:2.6

Op asked; "If God is merciful and loving and decides the fate of every human, why would he send them to hell? Why doesn't he send them all to heaven since he makes that final decision?"

There will be many judgments, and His anger does not last forever. In the end all will be reconciled back to Him as scripture declares.

and through Him to reconcile all things to Himself—having made peace through the blood of His Cross—through Him, whether the things on the earth, whether the things in the heavens. Col.1:20

He's working towards a final purpose and in the end all will only not only be Christ but God all in all. 1: Cor. 15:28

That will be heaven for the entire creation, death, sin, evil, gone. Behold the Lamb of God that takes away the sin of the world!

So the servants of the householder came and said unto him, Sir, didst not thou sow good seed in thy field? from whence then hath it tares? He said unto them, An enemy hath done this. The servants said unto him, Wilt thou then that we go and gather them up? But he said, Nay; lest while ye gather up the tares, ye root up also the wheat with them. Let both grow together until the harvest: and in the time of harvest I will say to the reapers, Gather ye together first the tares, and bind them in bundles to burn them: but gather the wheat into my barn.

Matthew 13:27-30 KJV

'An enemy hath done this'.

Those who are chosen in Christ, were 'chosen in Him before the foundation of the world that they should be holy and without blame before him in love' Ephesians 1:4.

Before the foundation of the world, before the creation of humanity, before Adam transgressed, before sin was in the world - then were they chosen in Him.

Afterwards, an enemy sowed the evil seed.

'An enemy hath done this'.

And these are to be left until the harvest of the whole earth when the vine of the earth is gathered :

And the angel thrust in his sickle into the earth, and gathered the vine of the earth, and cast it into the great winepress of the wrath of God. [Revelation 14:19 KJV]

'Bind them in bundles to burn them : but gather the wheat into my barn'.

John Calvin saw the spirituality of election. He saw what occurred before the foundation of the world. And he saw that when sin entered into the world, it was the deed of an enemy.

Thereafter, a seed was generated which was not of Christ.

'An enemy hath done this'.

As it is not by creation that God makes his elect, who have been tainted with original sin, to become a good seed, but by regenerating them through the grace of his Spirit; so wicked men are not created by the devil, but, having been created by God, are corrupted by the devil, and thrown into the Lord’s field, in order to corrupt the pure seed.

John Calvin - as referenced by David Gowler of Oxford University in regard to the parable of the wheat and the tares