Oversimplified, it takes more light to create the perception of yellow.
Yellow usually indicates the presence of both red light and green light in equal amounts, with a relative absence of some blue light. Another way of looking at it is that yellow is white that is missing a little blue.
Red light alone can create the perception of red. Same with green and blue acting alone. But the perception of yellow requires the perception of both red and green and often only a small drop in the perception of blue.
The reason behind this is that the human eye has no receptors dedicated to perceiving yellow light. We can only perceive red, green and blue directly. The other colors are a creation of our brains. This is how TV can generate a full range of colors from only those three primary colors. Yellow is red + green in balance. Orange is both red and green with more red than green.
There is one other way of creating the perception of yellow and that is with the wavelength of light where our receptors for red and green overlap in their sensitivity. Thus, this one color can stimulate both our receptors for red and green the same amount. This is how yellow laser light works. It is one monochromatic light source but our brain only knows that both our red and green receptors are stimulated by it.