Why fusion reactor is not made of high-temperature superconductor? As we know, there is several kind of high-temperature superconductors discovered. However, when I learned that current tokamaks and stellarators just mainly use Niobium-tin superconductor (which is mainly operated in 4.2K), I was surprised.
As I know(or believe), higher critical temperature allows more current density, allowing more powerful magnet, thus we can have higher confinement.
Why current scientists are not using the high-temperature superconductors for fusion reactor?
 A: Using High-Temperature Superconductors (HTS) appears to be a natural choice for magnetic confinement. Their usage in fusion reactors was proposed in the early 2000s but the major problem has been with the production scale. For example, REBCO. But now with new advances, we have capabilities to rely on HTS like REBCO which produce stronger magnetic fields than conventional $Nb_3Sn$. In fact, the SPARC collaboration aims at using these new advancements in HTS to build future fusion technologies with more gain $Q>1$ (or to break the $Q\sim1$ limit). For a recent review, please see "Overview of the SPARC tokamak", J. Plasma Phys. (2020), vol. 86, 865860502.
We need a large confinement time to have high fusion which implies we need large magnetic fields. You can also find information on one of the chapters of Superconducting Magnets in Fusion Reactors, R.G. Sharma, Springer Series in Materials Science, vol 214 (2021).
So usage of superconducting coils has been an active area of research, both on the material science aspects, their applications in a fusion reactor, and also engineering aspects.
