Simple, because that’s what the Bible says 😉 . Seriously, there are lots of nuances and wrinkles that make up the premillennialists argument; which I am not going to try and itemize here. I think one of the main determining factors for which millennial route one takes is in many ways determined by how an interpreter understands the relationship between Israel and the Church. Beyond that, hermeneutically I think how one deals with the genre of apocalyptic in the Bible will also have a drastic impact upon the shape of one’s millennial perspective. Not only that, but how an interpreter sees the New Testament and Old Testament relating to each other; this will also be a key determiner of one’s eschatological viewpoint.
For me, I see Israel and the Church as distinct yet inseparably related to each other within the framework of Yahweh’s unfolding redemptive plan. I see the genre of apocalyptic especially as used within the book of Revelation (i.e. taking its cue from its referent in the Old Testament), as functioning in a rather sequential past, present, future dialectic so that when we finally get to Revelation 20 (and the infamous thousand year passage) I believe that it is actually referencing an actual “future” intermediate stage of the in breaking of God’s kingdom in Christ. As far as the relationship between the Old Testament and the New Testament, I see a rather complex grid of interpretation taking place that cannot, given its complexity, serve as a normative pattern for 21st Century exegetes. Given all of the above, I am a premillennialist. I believe in a two-stage coming kingdom contra the one stage that an amillenialist would forward. (Of course I do hold to an ‘now/not yet kingdom view contra classic dipyism)