If you have monochromatic red and green light sources, like in a laser projector, you can reproduce very well the red-yellow corner, as mentioned by others.

However, in CRT displays, the color of the light emitted by the red phosphor was rather impure, far from a saturated red.

This limitation has been inherited by the sRGB color space, and because of this, the main defect of sRGB is that it cannot reproduce a lot of colors in the yellow-orange-red-purple corner.

This is very noticeable, because there are a lot of natural objects with such colors, e.g. flowers, fruits, birds, insects, clothes, whose colors appear washed out in sRGB, but they look much better on displays with greater gamut, like P3-D65 (Display P3) which is available in better monitors.

While the colors in the cyan corner cannot be reproduced well even with a laser projector, that is usually less objectionable than the poor reproduction of yellow to red colors by sRGB monitors, because interesting cyan objects are more rarely encountered (though they exist, e.g. certain gems, lichens, algae, insects, lizards and fish, certain clothes, frequently the littoral sea).