The problem with using nukes, is that you’ve just dropped a nuke. It doesn’t matter if it’s sub-kiloton, you’ve initiated first use of nuclear weapons in the conflict, and the other side will now likely respond with their own nukes.
Yeah. You’d have to be pretty desperate to want to use a nuke. But the low yield character of it makes it more likely you’d use it.
I suppose part of the calculus is that they want to maintain our defensive posture as cheaply as possible. Nukes are cheaper than soldiers and tanks.