Thats because Truth is one of the higest values in Christianity. Jesus emphasizes this by saying "I am the Way, the Truth, and the Life." Conversely, Satan's chief attribute in the Bible is that he is a liar.

That is also part of what makes the Ten Commandments different. Most of the commandments are no terribly unique. Many societies had prohibitions against theft, murder, and adultery. However, "Thou shalt not bear false witness against thy neighbor." was not seen in other societies' moral codes.