There seems to be two related issues raised with this question; a reason for Satan being bound, and when this happens. The reason is given in the text in question. Verse 3 clearly says:
"And cast him into the bottomless pit, and shut him up, and set a seal
upon him, that he should deceive the nations no more, till the
thousand years should be fulfilled: and after that he must be loosed a
little season." Revelation 20:2 A.V.
That answers the first part of the OPs question, "So why is Satan bound...?" He is bound in order to stop deceiving the nations as he had been doing, prior to being bound for "a thousand years" in the bottomless pit.
But the second part of the OPs question shows why there seems to be a problem, "...if the world from the last bowl judgment is [sic. - has] destroyed the earth completely?" Ah, well, if that is a misunderstanding about sequence of events, that would need to be cleared up. The OP knows that the Revelation is not in chronological order, so this is where two things Jesus said might help bring clarity.
"And if Satan rise up against himself, and be divided, he cannot
stand, but hath an end. No man can enter into a strong man's house,
and spoil his goods, except he will first bind the strong man; and
then he will spoil his goods." Mark 3:26-27 A.V.
"And the seventy returned again with joy, saying, Lord, even the
devils are subject unto us through thy name. And he said unto them, I
beheld Satan as lightning fall from heaven." Luke 10:17-18 A.V.
Does that not link in with Revelation 12:7-17 when, after the symbolic woman births the male child, Satan's attempt to destroy this one (Jesus Christ, surely?) fails due to the archangel Michael casting Satan, "who deceives the whole earth" (vs.9) down to Earth. Verse 12 adds that Satan knows he then has "but a short time", and wages war against the remnant of the woman's seed (vs. 17). Satan is greatly restricted and confined, yet is able to orchestrate persecution against those who "keep the commandments of God" on Earth. This happens before the final Day of Judgment with its resurrection of all the dead.
This also links in with Revelation 20:1-3. They are not separate events and different times, for the Revelation keeps repeating the sequence of events starting with Christ returning in glory to heaven, to be enthroned there, till the Day of Judgment. Chapter 12 begins the 4th repeating sequence of this (adding more details to what was stated before), and chapter 20 starts the 7th repeating sequence. This enables a build-up of understanding, more details being added to what has already been revealed. This is explained in this book:
"The seventh and last opening brings the Book of the Revelation of
Jesus Christ to a perfect consummation. Beginning at chapter 22, this
breathtaking series of visions - each in itself astounding - as a
whole overwhelms the reader. There passes before the eye the prophetic
sequence of the judgment of the dragon; the first resurrection; the
Last Judgment, the passing away of the earth and of the heaven; the
second death; the bringing in of a new heaven and a new earth; the
revelation of the holy city; and the vision of the everlasting
glory...
The first two verses show the commencement of the thousand years;
verses 3-6 narrate the continuation of the thousand years; and verses
7-10 declare the conclusion of the thousand years. The final verses,
11-15, depict the Last Judgment at the end of the world." The
Revelation of Jesus Christ, pp/535-6, John Metcalfe (published 1998)
http://www.johnmetcalfepublishingtrust.co.uk/contact_us.htm for
details of the 2022 reprint.
Then it explains why the thousand years cannot be taken as a literal number; it is symbolic of the whole time from Christ's glorious ascension to heaven until his return. That fits in with what Jesus said in the verses quoted above.
"...are we asked to believe by those who take such numbers literally,
in realms and dispensations beyond known time, invented by
premillennialists and dispensationalists for the purpose of
rationalizing the irrational, that the thousand years of Revelation
20:2 is a cold, factual measure of time? But we had been told by the
Holy Ghost 'that there should be time no longer.' Chapter 10:6. If so,
how can a period subsequent to the cessation of time be measured by a
period dependent upon its existence.
Not to say that in the old testament the only intimation of a thousand
years belongs within the compass of time, Psalm 90:4, and in the new,
other than Revelation 20, the sole reference to a thousand years of
time is applied by the apostle Peter [2 Peter 3:8] prior to the
second coming of Christ and the dissolution of the heavens and the
earth." (Ibid. p537)
Revelation 20:1-2 is taken to mean the commencement of the (non-literal) thousand years, chapter 12 being examined to show that the dragon was defeated, and cast out of heaven following the birth and ascension of Jesus Christ:
"This was at the present age, after Christ, made of a woman, made
under the law, came to 'redeem them that were under the law, that we
might receive the adoption of sons', Galatians 4:4,5. This was when
Christ blotted out 'the hand-writing of ordinances that was contrary
to us, and took it out of the way, nailing it to his cross; and having
spoiled principalities and powers', that is, by casting them out of
heaven - 'he made a show of them openly, triumphing over them in it' -
namely, in the cross - see Colossians 2:13-15.
This answers to that which is written, 'Forasmuch then as the children
are partakers of flesh and blood, he also himself likewise took part
of the same; that through death he might destroy him that had the
power of death, that is, the devil; and deliver them who through fear
of death were all their lifetime subject to bondage', Hebrews 2:14,15.
This destruction of the devil, casting him powerless out of heaven -
as it were, bound - took place because through death Christ delivered
his people from the law and its curse, robbing the accuser of the
brethren not only of the strength of his legal case, but also of any
lawful right to make a case at all. Why? Because deliverance from the
law took the accused out of the courtroom for ever. Thus Christ fought
against the accuser, prevailed over him, cast him out of heaven, and
brought him down to earth. This was the position in which the dragon
was seen at the close of Chapter 12." (Ibid.) p.539
This lengthy sequence of quotes has been necessary to clear up the second part of the OPs question. Conclusion: Satan is confined, bound, from Christ's ascension until a short time before Christ returns, so that Satan then has another go at deceiving the nations. But as Revelation 20:10-15 shows, Satan and his minions are cast into the eternally burning lake of fire and brimstone, followed by the Last Judgment, where his earthly followers are judged and join them. After that comes the new heaven and the new earth (21:1 on).