The START II treaty limited Russia and the US's ICBMs to a single warhead each, and the Peacekeepers were optimised as a platform to host multiple independently targetable re-entry vehicles (MIRVs) and when the US agreed to revert to a single warhead per missile the Minuteman III was much cheaper to maintain than the Peacekeeper.

So even though Russia withdrew from START II almost immediately, the US continued to unilaterally remove the MIRV capability from its ICBM fleet and stick to single warhead Minuteman IIIs.