I understand the economic incentives for adopting widespread slavery in ancient time periods, such as how they were employed by the Roman Empire or the Athenians. Cheap and plentiful labor (often from prisoners of war, or civilians of conquered enemies) to build a stronger economy than your geopolitical rivals in a world where the strong conquer the weak.

What I don't understand is why slavery became less important for the success of feudal societies in Europe during the medieval age, and why serfdom became preferable.

What were the reasons that slavery (employment of slaves and the slave trade) eventually became less widespread in Europe during the medieval periods and could be replaced by serfdom?

• What were the economic reasons? Why was cheap labor less necessary in a feudal economy than before, and why was serfdom chosen as the ideal economic model?
• Were there any changes in religious and moral views towards slavery as well?
• Which was the more significant reason - economic or religious/moral trends?

Also, after the late middle ages, slavery was prominent in colonies but not in Europe. Why was that?