Except that 1) that's because it was used to diagnose leprosy by the priests and 2) it was unnatural to the Jews.
We are Gentiles (most of us), and are not saved by levitical law, but through Christ's sacrifice. The priesthood is no longer in the position to diagnose PHYSICAL contagions because 1) we have full time physicians and 2) Christ is our high priest and great physician.
I'd wager (although I can't say for certain) that it may be taught for the same reasons that some churches say that any cosmetics are sinful: they see it as a sign of immodesty and vanity.
(Personally, I don't feel that way. Romans 14 says we have all liberty... But Paul also says that if someone else feels convicted, then honour their conviction.)