Sam Storms is an ex-dispensationalist, has written a very good book on why amillennialism is a better option than premillennialism, including dispensationalism, and here is an excerpt that gives some very good reasons:
Scriptural Challenges for Premillenialists
Sam Storms
If you are a premillennialist, whether dispensational or not, there are several things with which you must reckon:
• You must necessarily believe that physical death will continue to exist beyond the time of Christ’s second coming.
• You must necessarily believe that the natural creation will continue, beyond the time of Christ’s second coming, to be subjected to the curse imposed by the Fall of man.
• You must necessarily believe that the New Heavens and New Earth will not be introduced until 1,000 years subsequent to the return of Christ.
• You must necessarily believe that unbelieving men and women will still have the opportunity to come to saving faith in Christ for at least 1,000 years subsequent to his return.
• You must necessarily believe that unbelievers will not be finally resurrected until at least 1,000 years subsequent to the return of Christ.
• You must necessarily believe that unbelievers will not be finally judged and cast into eternal punishment until at least 1,000 years subsequent to the return of Christ.
So what’s wrong with believing these things, asks the premillennialist? What’s wrong is that these many things that premillennialists must believe (because of the way they interpret Scripture), the NT explicitly denies. In other words, in my study of the second coming of Christ I discovered that, contrary to what premillennialism requires us to believe, death is defeated and swallowed up in victory at the parousia, the natural creation is set free from its bondage to corruption at the parousia, the New Heavens and the New Earth are introduced immediately following the parousia, all opportunity to receive Christ as savior terminates at the parousia, and both the final resurrection and eternal judgment of unbelievers will occur at the time of the parousia. Simply put, the NT portrayals of the second coming of Christ forced me to conclude that a millennial age, subsequent to Christ’s return, of the sort proposed by premillennialism was impossible.
https://www.thegospelcoalition.org/article/why-i-changed-my-mind-about-the-millennium/
https://www.thegospelcoalition.org/...u-must-believe-if-you-are-a-premillennialist/