Is it specifically the emissivity that's so unusual and makes it the preferred material? I don't know that "low emissivity in infrared + high emissivity in visible" is a rare property—I recall reading about space satellite thermal engineering and seeing long lists of common materials sorted by visible/infrared emissivity ratios [0]. And there are a lot of them, in every category. I suspect the key thing is ThO2 is a super-refractory with a melting point of (approximately—these are hard to measure!) 3,350° C [1]. (About the same as the tungsten filaments in the old-school type of lightbulbs—something with pretty similar considerations). I suspect "things that maintian structural integrity in hot gas flame" is really the key discriminator here, the rare property that prunes out most candidates.
I'm not any sort of expert on this, to make very clear! Just a curious geek.
[0] (That's basically a proxy for the radiative equilibrium temperature in space: visible emissivity measuring absorption of sunlight, infrared emissivity measuring emission of waste heat. (To those unfamiliar, absorption and emission are exactly the same, at a specified wavelength: the physics is reversible). ThO2 for example, you'd expect would get extremely hot in space).
