For many of the same reasons that Spacex is using it for their starship. It's cheap and readily available, relatively easy to work with, and very well studied. There also really are not that many materials that have passed certification for use in reactors. I believe that "BPVC Section III-Rules for Construction of Nuclear Facility Components-Division 5-High Temperature Reactors" only covers half a dozen stainless steels.

As for cladding, the DOE's 2021 budget includes money for testing novel claddings. They are also looking for robust redox reference electrodes and additive manufacturing methods.