Why is sexual dimorphism rare in monogamous species?

Answer 1

When species are monogamous, males and females often pair up equally. The males don't have to compete as severely and so don't evolve traits to fight or attract females. There would be little benefit for a male to evolve energy-consuming dimorphic traits if they have fairly easy access to female mates.

Answer 2

To put it briefly, sexually dimorphism exists as a function of competition from the dominant sex. For instance, in hyenas the female is the dominant sex. The larger the female, the fitter she is to fight other females for mates, have success in hunting, survive periods of stress like drought or famine. The phenomena is rarer in monogamous species because it lacks competition for multiple mates. The big female hyena wants to copulate with as many males as possible ensuring genetic persistence. However, there isn't the need to constantly outcompete for sexual mates like that in humans. Human males are not constantly thumping each other win the right to impregnate several women.

The REASON why we observe this is that the strongest and most fit (e.g. the hyena again) was then able to transmit genetic instructions to make BIIG females in may different partnering's. I hope i helped!