Why do simulations of dichromatic color vision portray medium wavelengths as yellow rather than green?
Opponent color processing. 

https://en.wikipedia.org/wiki/Opponent_process

Studies have shown blue and yellow are opponent colors, as are red and green. We process the colors in opposition, as in perceiving one requires perceiving less of its opponent, equal amounts of both (minus other colors) yields no color in your final visual perception. If you cannot differentiate red and green, than the only two colors you can differentiate reliably are yellow and blue. As a side note this is also why in image processing programs these colors are on paired sliders, it is the simplest way to manipulate color images. 

The most common forms of color blindness in humans, and the one all non-primate mammals have prevents them from differentiating red and green. 

If you cannot differentiate red and green, and if I show you blue and yellow, you see no color, if I show you red and green you see yellow, becasue your blue cone is not being activated. 

The image below is our opponent pairs in our vision.