Cite, please.

200 Soviet nukes lost in Ukraine -- article from Sept 13, 2002. There have been reported losses of nuclear submarines at sea since then as well (though those are improbably recoverable). Note: even if that window is closed now, it was open then, and no terrorist groups used that channel to acquire nukes -- nor is there, as your citation notes, even so much as an actually recorded attempt to do so -- in the entirety of that window of opportunity.

When dozens of disparate extremist groups failed to even attempt to acquire a specific category of weapon, we can safely at that point generalize into a principle that governs how 'terrorists' interact with 'nukes' (in this case) such that they are exceedingly unlikely to want to do so.

In this case, I assert it is because all such groups are inherently political, and as such the knowable political fallout (pun intended) of using a nuclear bomb is sufficient that it in and of itself acts as a deterrant against their use: I am possessed of a strong belief that any terrorist organization that used a nuclear bomb would be eradicated by the governments of every nation on the planet. There is no single event more likely to unify the hatred of all mankind against the perpetrator than the rogue use of a nuclear bomb; we have stigmatized them to that great an extent.