Why can't a civilian own a M4 carbine?

A civilian cannot own an M4 carbine because it is classified as a military firearm and is subject to strict regulations under the National Firearms Act (NFA) in the United States. The M4 is a fully automatic weapon, and civilian ownership of fully automatic firearms manufactured after 1986 is prohibited. While civilians can own semi-automatic variants of similar rifles, the specific M4 designation and its features are restricted for military use only.