they don’t launch missiles from dedicated launch sites or missile silos in the ground anymore, they use stealth aircraft or fighter jets to carry the missiles for hypothetical first strike scenarios. airbases that close to russia could let them attack the capital before they could meaningfully retaliate. rando military officers promoted via emergency aren’t going to be as willing or able to push the big red nuclear retalliation button as the career politicians and generals in the capital. ICBMs like the ones carried by nuclear subs are easier to detect and intercept (or more realistically retalliate against the launcher), because they have to reach high altitudes to fly with less wind resistance, while nukes deployed via even normal un-stealthed aircraft can be camouflaged more easily, as they don’t have to have the range or size of ICBMs. they don’t necessarily know just from radar if its a nuke on the plane and not a normal missile, for example, and weapon systems like low-altitude cruise missiles launched from planes relatively closeby to the target could take a path through terrain that would conceal it from radar by using treetops and mountainlines as cover.
and you don’t think they would use that “small part” of ukraine? you don’t think having access to ukrainian airspace, ideally someday without russian military euipment within its borders, would be helpful at all? like maybe we would want to launch a multi-pronged attack from several locations at once or something?
no one said it was the only reason, there’s also the consistently broken ceasefires and ethnic cleansing of russian speakers in the donbass and luhansk republics. and the american interference in ukraine’s government. see any UN report from before 2020.
