[dane-pgp]

> why did they want to cede sovereignty to the EU so bad?

Signing a treaty (which you can later leave) is not a loss of sovereignty, it is an exercise of sovereignty. If you want to be reductionist about it, a country joining NATO would be a loss of sovereignty too, because they would be committing their foreign and military policy to a supranational body.

Similarly, your proposal of Ukraine promising not to join NATO (presumably as a condition in some treaty?) would also be a loss of sovereignty, by that logic, because a truly sovereign nation is entitled to apply for NATO membership whenever it wants to.