Why do Premillennialists teach the 1,000 year reign of Christ will take place on Earth? Revelation 20 [1] mentions a special 1,000 year period commonly referred to as the Millennial Reign. That chapter also mentions of an angel coming down from heaven. However, the Holy City does not come down until Revelation 21 [2]. It makes me wonder if the Millennial Reign will take place in Heaven. The main reasons are:

There is no mention of Millennial Reign taking place on earth.
Where would the Millennial Reign be on earth if the Holy City has not landed on earth until Revelation 21.

But Premillennialists teach that this Millennial Reign will take place on earth. What is the Biblical or theological basis for their teaching?

[1] Revelation 20:1 - 6 ESV 
Then I saw an angel coming down from heaven, holding in his hand
the key to the bottomless pita and a great chain. And he seized the
dragon, that ancient serpent, who is the devil and Satan, and bound
him for a thousand years, and threw him into the pit, and shut it and
sealed it over him, so that he might not deceive the nations any
longer, until the thousand years were ended. After that he must be
released for a little while. Then I saw thrones, and seated on them
were those to whom the authority to judge was committed. Also I saw
the souls of those who had been beheaded for the testimony of Yahusha
and for the word of Elohim, and those who had not worshiped the beast
or its image and had not received its mark on their foreheads or their
hands. They came to life and reigned with Christ for a thousand years.
The rest of the dead did not come to life until the thousand years
were ended. This is the first resurrection. Blessed and holy is the
one who shares in the first resurrection! Over such the second death
has no power, but they will be priests of God and of Christ, and they
will reign with Him for a thousand years.


[2] Revelation 21:1 - 3 ESV 
Then I saw a new heaven and a new earth, for the first heaven and the first earth had passed away, and the sea was no more. And I saw the holy city, new Jerusalem, coming down out of heaven from Elohim, prepared as a bride adorned for her Husband. And I heard a loud voice from the throne saying, “Behold, the dwelling place of Elohim is with man. He will dwell with them, and they will be His people, and Elohim Himself will be with them as their Elohim
There is no mention of Millennial Reign taking place on earth.

Not true.
When Christ returns, the tribulation will end and he will become King of Kings, here on Earth:

Blessed are the meek, For they shall inherit the earth. — Matthew 5:5


And have made us kings and priests to our God; And we shall reign on the earth.” — Revelation 5:10


The kingdoms of this world have become the kingdoms of our Lord and of His Christ, and He shall reign forever and ever! — Revelation 11:15

The Old Testament contains many prophecies about the world to come, and it clearly takes place here on Earth.
At first many nations will refuse Christ's rule, but soon they will all come to accept it and cooperate with each other during the Millennium:

And it shall come to pass that everyone who is left of all the nations which came against Jerusalem shall go up from year to year to worship the King, the LORD of hosts, and to keep the Feast of Tabernacles.
And it shall be that whichever of the families of the earth do not come up to Jerusalem to worship the King, the LORD of hosts, on them there will be no rain.
— Zechariah 14:16–17


He shall judge between the nations,
And rebuke many people;
They shall beat their swords into plowshares,
And their spears into pruning hooks;
Nation shall not lift up sword against nation,
Neither shall they learn war anymore.
— Isaiah 2:4


The wolf also shall dwell with the lamb,
The leopard shall lie down with the young goat,
The calf and the young lion and the fatling together;
And a little child shall lead them.
— Isaiah 11:6


In that day there will be a highway from Egypt to Assyria, and the Assyrian will come into Egypt and the Egyptian into Assyria, and the Egyptians will serve with the Assyrians.
— Isaiah 19:23

Would there be any need for plowing and pruning, or highways and rain, in Heaven?
Will there be wolves and leopards and lions? and children?

Where would the Millennial Reign be on earth if the Holy City has not landed on earth until Revelation 21.

The Millennial Reign will be over the entire earth (it's not clear why you would think otherwise).