Ukraine had no alternative.

We have various analyses in the National Security Archives, reports and memoirs from US diplomats involved in the matter: the US was the primary actor that pressured Ukraine in giving up their nuclear stock pile.

Ukraine could not operate it (everything from security, cryptography, controls, navigations, etc was in Moscow anyway), could not maintain it, and could not even guard their stock pile.

The biggest risk for the United States was that with the rampant corruption and sell off of soviet weaponry on the black market there were the wrong incentives for too many actors that could bring soviet nuclear weapons in the hands of bad actors.