There's probably some optical reason.
In the absence of knowing what that might be, I dare to guess: orange arrives at the eye sooner than indigo? there's less orange in the typical background? it has some innate charisma?
My hope is that when I figure it out, I'll know something about forthrightness that I didn't know before.
My research turns up the following:
About.com says the color is mentally stimulating, less aggressive than red, sociable, and associated with change.
Color expert Kate Smith says orange is more controversial than any other color; people have strong feelings about it one way or another. Also, it's fun and flamboyant. (I think we know what side she's on.)
Orange is the color linked with the second chakra, which is in turn associated with creativity and sex, happiness and courage.
Got anything orange in your closet?