In other words, why is it so hard to get the Q value above 1 (ration between energy output and input)
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Nuclear fusion takes a lot of input energy to get it going. It also takes a lot of energy to keep a fusion reaction going. But fusion reactions also give off a lot of energy.
The difficulties with efficient reactors are getting enough energy focused into one spot in the first place, and then getting enough of the output energy fed back into the reaction to sustain it (while providing new fuel, and extracting the excess energy in a useful way).
Stars are the standard nuclear fusion reactors. For them, gravity provides the input energy; the gravity of the Sun is enough to squish hydrogen together hard enough to get it to fuse. That is a lot of energy, though.
Jupiter has 300 times the mass of the Earth but isn’t massive enough to get nuclear fusion to start.
Without gravity nuclear fusion usually involves focusing a lot of energy into a single spot. The US’s National Ignition Facility, for example, achieves nuclear fusion by [focusing a whole bunch of giant lasers](https://upload.wikimedia.org/wikipedia/commons/3/3d/NIF_building_layout.png) onto a single spot (the circle in the bottom right of that image). NIF ran from 2009 to 2012 but even with all those lasers wasn’t able to focus enough energy in a small enough spot to get nuclear fusion to start.
It was redesigned and started up again in 2021, and managed to achieve nuclear fusion.
It is really difficult to focus all that energy in one place to start the reaction. The reactor then has to achieve the same effect with the random energy the reaction throws out. It is much easier to focus energy you are in control of (from the lasers) than from the nuclear explosion you just created. Think of it as the difference between lighting a candle with a match and lighting it with a bomb. In theory it could work, but the bomb energy is much harder to control and focus.
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