Not a physicist, I'll take a crack at it. Fusion needs two things: 1. plasma (easy part) 2. pressure (hard part) - this is normally referred to as confinement, which is misleading. What you're trying to do is squeeze the plasma into a really small volume, which is like squeezing water in your hands into a perfect compressed sphere. Try it.

Stars do it easily, because the confinement / pressure is provided by immense gravity which very conveniently works to compress a huge load of gas into a point space.