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EMP weapons are detonated at a range of altitudes between 60 to 300 km. If the NEMP detonation takes place in the ionosphere, the resulting EMP is spread over such a large area that it can span an entire continent.

Its the altitude IMHO. You need to detonate the weapon at high-altitude if you want to affect a large area. So Nukes do not achieve all that an NEMP weapon does.