When He reigns again, it will be for eternity, not some mean little chunk of a thousand years.
The thousand years and eternity are not mutually exclusive.
The book of Revelation specifically mentions the thousand years six times. The thousand years are important enough for the Holy Spirit to mention the thousand years six times in two chapters.
As far as that goes, what about the 6000 years man has been here (according to the Bible)? What is 6000 years compared to eternity? Nothing. Yet the Bible is devoted almost entirely to that 6000 years. Christ's reign will make it 7000 years. What happens here on earth including Christ's 1000 year reign must be very important to God.