Ukraine couldn't have kept nuclear weapons. It needs a lot of technical expertise to do that, particularly in today's world where you only test them in simulation which means you need great ability to trust your simulations. Ukraine didn't even have the keys to use the weapons they had (Russia did) which means they needed to first rebuild each with new keys. Not that Ukraine couldn't do all that, but they just don't have the money to do that and everything else they also need to do. Nuclear weapons are an obvious first thing to go because they are only useful in a situation where you want to end the world. In almost all cases it is better to be able to defend yourself without ending the world.