Other than the fact that venus is a goddess and mars is a god, I think the main reason was a marketing decision. The book which coined the expression was originally titled:\n\n"The Women are from Venus, the Men are from Uranus"\n\nBut then again, they thought it might be subject to some juvenile pranks. So they switched to Mars. Which is closer to earth, but not as close to the point as Uranus.
why did they say men are from mars and women are from venus?