Because one of the theories (evolution) is a scientific theory, and is therefore appropriate for science class, like atomic theory, germ theory, and the theory of gravity for example. The other theory (ID) was admitted to be unscientific on the stand by ID's leading scientific proponent, Dr Behe (who also agreed under oath that evolution *is* a scientific theory). Dr behe admitted under oath that if the standards of scientific theories were lowered to admit ID then astrology would have to be considered a scientific theory also.
Teach ID in church, and evolution in science class.
Let's see, you say one is a theory and the other is a theory! Where do you stand on theories? They are just that....theories! Nothing absolute on either count!