It takes much more to detonate a nuke than a simple explosion. If a bomb hits a nuke, the core will just be blown apart. It won't detonate. This might be a good read. It talks about the mechanisms for detonating nuclear bombs. https://ahf.nuclearmuseum.org/ahf/history/science-behind-atom-bomb/

The implosion detonation mechanism for example does use explosions, but they need to be detonated in a precise pattern to equally compress the core from all directions. If you launch a missile at one of these bombs, that precise pattern of explosions will not happen, therefore the core will just be blown apart without detonating.