Why is it prohibited for a Muslim man and women to have a relationship outside of marriage?

Any relationship out of marriage is strictly prohibited. If both parties are not legally committed by shariah to one another there is a potential of abusing one another by merely fulfiling their carnal desires and thereafter abandoning one another. The matter can become more complex if the female becomes conceived in this illegal relationship, which is followed by the question of abortion, illegitimate children, abandoning such children, etc.

And Allah Ta'ala knows best

Mufti E. Desai