Because to achieve 1MW of energy from fusion, you put 200MW of power into the lasers. And, you have to fire these lasers with unfathomable precision at a tiny piece of gold, in order to heat up an even tinier pellet of hydrogen, which then heats up enormously for a few milliseconds before fizzing out.
ICF is a good way of studying what happens inside a hydrogen bomb, but it is in no way imaginable how you could use it as a weapon in itself. At this point, you'd be much, much better off just firing the lasers at your target (though even that wouldn't achieve much, unless you target is kind enough to step in front of a highly sensitive, gigantic laser).
Edit: corrected a typo graciously pointed out by GP.
What stops you from using the "even tinier piece pellet of hydrogen"[sic] from initiating fusion in a slightly less tiny pellet of hydrogen that it's sitting on top of, which initiates fusion in a slightly less tiny pellet of hydrogen, and so on? Aside from concern for your own survival, of course.
Maybe if you can't fathom the precision required to irradiate the NIF hohlraum sufficiently isotropically to achieve ignition in the first place, you shouldn't be trying to answer this question.
Nuclear fusion requires both compression and heating of the fuel. In nuclear weapons, this is accomplished with a combination of radiation pressure and a fissile sparkplug, respectively. In inertial confinement fusion, there are two distinct laser pulses with different characteristics. A fusion pellet detonating would release radiation that could compress another pellet, but there would be no method of heating that pellet at the appropriate moment.
There may be an engineering method to overcome this, but it would be way beyond the difficulty of getting that first pellet to ignite, which already is a bleeding edge technological development.
> What stops you from using the "even tinier piece pellet of hydrogen"[sic] from initiating fusion in a slightly less tiny pellet of hydrogen that it's sitting on top of, which initiates fusion in a slightly less tiny pellet of hydrogen, and so on? Aside from concern for your own survival, of course.
The same thing that stops you from igniting the initial pellet with the hohlraum - you don't have anything creating the kind of confinement necessary to keep the plasma together.
The only thing allowing the plasma to get hot enough for fusion is the initial velocity of the inward-spreading shockwave from the initial explosion of the outer shell of the pellet. As the velocity of this shockwave inevitably decreases, confinement is inevitably lost and the plasma dissipates and cools down.
Probably in principle you could use the energy of the first pellet's plasma to cause similar shockwaves in a second, larger pellet and so on, but that requires an entirely different geometry, its not just a matter of putting the second pellet close to the first one.
Between the primary fission bomb and the secondary fusion bomb there is a huge shield, so the shockwave of the first one hit's the second one at the same time everywhere, instead of hitting the top.
My guess is that to put a ternary fusion bomb you will need another even bigger shield, but IANANBS.
ICF is a good way of studying what happens inside a hydrogen bomb, but it is in no way imaginable how you could use it as a weapon in itself. At this point, you'd be much, much better off just firing the lasers at your target (though even that wouldn't achieve much, unless you target is kind enough to step in front of a highly sensitive, gigantic laser).
Edit: corrected a typo graciously pointed out by GP.