More curved means more optical aberrations. The Rubin is a so-called three-mirror anastigmat design that minimizes astigmatism, coma, and spherical aberration. (Chromatic aberration is not a problem in reflectors because dispersion only occurs when light is refracted.) A two-mirror design couldn’t be used in such a wide-field telescope without severe image quality issues, at least comparatively speaking.
https://en.wikipedia.org/wiki/Three-mirror_anastigmat