My understanding is that one big reason so much money was invested in this lab is because the research has direct applications to nuclear weapon design. It's a dual use lab so to speak. The process they are studying is very similar to what's happening in the core of a thermonuclear bomb.
> 1978: This report reviews aspects of the military applications of the inertial confinement fusion (ICF) program at Sandia Laboratories
> Today, research on inertial confinement fusion—the other leading approach—remains largely under the control of US national weapons labs. The military focus has had profound impacts on the development of inertial fusion energy.