The lack of a terrorist nuclear attack is hardly remarkable. There have only been a few "broken arrow" incidents. Most of them would have caused major damage to the physics package and other key components. Or everything is deep under the ocean, beyond scuba range.

Even if a terrorist somehow obtained a complete warhead in good condition they would have a limited time window to use it. In order to operate reliably, warheads require regular maintenance by skilled technicians, and then every few years the entire thing has to be pulled and remanufactured. This is part of the reason why our nuclear deterrent is so enormously expensive: there's a lot of support infrastructure behind the weapons and delivery platforms.

A radiological weapon (dirty bomb) is a more realistic threat.