Why do most two-element Yagis with normal spacing have a reflector instead of a director?
Why In a two-element parasitic array the parasitic element can be tuned either inductively (longer, a reflector) or capacitively (shorter, a director). At the spacings normally used for two-element beams, roughly 0.15 to 0.25 wavelength, the reflector arrangement produces more forward gain than a director at the same spacing, and it also gives a higher feed point impedance and wider usable bandwidth. A director only becomes competitive at very close spacing, around 0.1 wavelength, where the radiation resistance drops sharply and the bandwidth becomes very narrow.
Watch out Front-to-back and front-to-side ratios in a two-element Yagi are actually modest and can be better with a closely spaced director, so pattern figures are not the reason builders choose a reflector; the choice about SWR confuses feed point impedance convenience with the main design goal.
Two elements, normal spacing: the reflector is the gain winner.