The smaller the nuke, the shorter the shelf life.The less shielding that you have, the sooner that your electronics and conventional explosives deteriorate from the radiation.
The less fissionable material that you have, the faster you generally need your atomic trigger isotopes to emit neutrons. The faster you emit neutrons, the shorter your half-life. The shorter your half-life, the less time that you have before the nuke simply fizzles instead of booms.
This is simple physics. Moreover, heavy metals like uranium and plutonium are among the most brittle materials known to man, and the slightest bit of humidity turns them into uranium oxide or plutonium oxide (i.e. worthless rust).
So a "suitcase nuke" from 1996 is likely little more than a rusted, shattered, fragmented collection of wiring and explosives today.
9 Legislative Days Left Until The AWB Expires