Because they are cultural treasures. Because they are sacred oral traditions. Because they hold concepts which the modern world is starving for, such as mitakuye oyasin (Lakota for all my relations).
From the BBC
QUOTE Language is the distillation of hundreds, if not thousands of years of experience of a collective... So when the language disappears you're really throwing away that whole library of knowledge UNQUOTE
Rachel Nez - Navajo speaker