There are two factors -- first, as you guessed, it's much easier to get a missile "close enough for a nuke" than it is to hit an ICBM. Second, nukes produce el

There are two factors -- first, as you guessed, it's much easier to get a missile "close enough for a nuke" than it is to hit an ICBM. Second, nukes produce electromagnetic pulses, which could disable ICBMs which are too far away to be destroyed completely.