“Explain in more detail while ISIS would use a nuke right away if they had it.”
The simple version:
The longer ISIS waits to use a nuke, the more likely it won’t work.
Batteries wear out.
Irradiated components degrade rapidly.
Radioactive components “wear out” on their own (half-life degradation).
Stupid people tend to break things.
And every major military in the region & world would be he11-bent on finding it and carpet-bombing the suspected location with extreme urgency.
And, rumor is, it’s really hard to hide one of these things (radiation-detecting satellites can basically see them all everywhere).
The Muslim way is to give warning, so those who are hit have a chance to choose Islam instead. If ISIS does have nukes and they need to use them very soon, the warning could mean that their use is imminent.