The stated rationale was to permit an orderly transition on the part of the nuclear arsenal and its supporting industrial establishment to a permanent no-testing environment.
Importantly, when such a permanent prohibition on testing - in the form of the Comprehensive Test Ban Treaty (CTBT) - was submitted to the U.S. Senate for its advice and consent in 1999, the same considerations that underpinned President Reagan's position on testing were among those that caused a majority of Senators to reject the CTBT.
President Clinton would be well-advised to learn from the experience of his Democratic predecessor, Jimmy Carter, who came to office determined to achieve a comprehensive, permanent ban on nuclear testing.
Ironically, this fact was explicitly recognized in the Hatfield-Exxon legislation of 1992 that led to the present moratorium -- legislation that expressly contemplated additional underground tests would be necessary to prepare the U.S. stockpile, diagnostic tools and scientific cadre for a permanent ban on nuclear testing.
This law also directed a moratorium on U.S. testing after up to fifteen tests were conducted to help prepare the American nuclear arsenal for a permanent cessation of underground detonations.