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Well lets look at this way...Captured Al Qadea computers in Afganistan have Nuclear Bomb making information on them. Which at the very least shows an interest in them.

The head of Pakistans Nuclear program went to Afganistan and had conversations with Al Qadea.

There are several Soviet Suitcase Nukes among the missing...as in not knowing where they are????

There are literally 100,000s of Tactical Nuclear weapons laying about in the Soviet Union, guarded by underpaid guards

There are numerous Soviet Nuclear experts who are underemployed or not employed...

I think the correct answer to the question is that Al Qadea is being disrupted enough by the hounds on their trail that they are not able to pull off a big job. The West realizes the threat is serious so we keep on turning over rocks.