It is because brown really isn't just dark orange. There are oranges that are both dark and not brown - these usually are on the redder side of orange, like dar

It is because brown really isn't just dark orange. There are oranges that are both dark and not brown - these usually are on the redder side of orange, like dark orange clouds in the sky at sunset. Brown is, in general, a muted orange. Take the red and yellow, add a hint of green (or purple, honestly) - and you have browns.