Because most toothpaste contains a very toxic substance: fluoride
Just how toxic is fluoride?
fluoriden. a salt containing fluorine (Websters New World Dictionary For Young Readers, Copyright 1983, 1979 by Simon & Schuster)
fluorine n. a corrosive, poisonous, pale greenish-yellow gaseous chemical element, the most reactive non-metallic element known, ... (Websters New World Dictionary, Third College Edition, Copyright 1988 by Simon & Schuster)