Why does the ITU-R only regulate lower frequencies up to some hundred GHz and not higher frequencies in the THz range?

Because of their higher reach. And thus, military applications and others should be free to use them regardless of the country or region where they are transmitted. Higher frequencies have a much lower reach, since their wavelength is lower, and so there's sense in regulating them.

Besides, lower frequencies implies lower data transmission rates, and hence its lower importance as well, I'd say!