Only the criminals are allowed to have them.
Check it out:

A notice from the Bureau of Alcohol, Tobacco, Firearms, and Explosives (ATF) shows the agency is considering a ban on the popular M855 AR-15 round — by re-categorizing the round as “armor piercing.”

Once categorized as “armor piercing,” sales of the M855 rounds can be restricted or banned altogether under language in the Gun Control Act of 1968 (GCA), which describes marketable ammunition as that which is “primarily intended to be used for sporting purposes.”

The ammunition guidelines set forth in the GCA are intended for handguns, but are also being applied to ammunition used in AR-15 rifles because AR-15 pistols fire the same ammunition.