In magnetic confinement fusion, power density scales to the fourth power of the magnetic field strength. The only way to reasonably achieve the kind of magnetic fields needed without massive constant power draw is superconducting magnets. For a very long time, the best superconductors available limited the maximum field strength to ~ 5-6T. This limited the space of possible magnetic confinement reactors into basically ITER, or "big enough that you need a major multi-decade international collaboration to build it".
Last decade, major discoveries were made in practical high-temperature superconductors. By 2017, you could commercially purchase superconducting tape that can do >8T, the state of the art has improved since, and keeps improving. All the constraints and limits on fusion that were known since the field began are changing now. Most importantly, with high-field magnets, the size of a theoretically functional net-energy fusion reactor comes way down, to what is achievably by a reasonably well funded startup. The whole field is open now, and all the different magnetic confinement fusion technologies that were tested and discarded over the first 60 years of fusion research are now being re-evaluated in ligth of the new magnets that are available. That's why there are now like a dozen well-funded fusion startups.