Mostly because of popular media
a lot of people believe that Teflon coated bullets
have superior armor piercing
And enough people have been misled about teflon coating that it is now illegal for non LE people to use and own these bullets in fear that it might be used against the police. In some states of the USA at least.
The myth stems from the fact that real AP rounds indeed use a teflon cap to protect its rather pointy ogive and facilitate feeding in an automatic firearm. The teflon itself doesn't do any armor penetrating, in fact teflon is so soft it can be used as a lubricant.
Teflon coated lead slugs are only made because it shoots cleaner and deposits less lead on the rifling and makes the barrel easier to clean after use. It flies just as fast as a similarly weighing lead bullet if put over the same powder charge. It also wears the rifling less than fmj rounds.
Teflon coated lead bullets usually are black.