Why is idolatry a serious sin?

Idolatry involves giving veneration or devotion to an object or cause that is not Jehovah God. He states in scripture that He desires "exclusive devotion" (Exodus 20:5), as well as commanded His followers to serve no other gods and to avoid idol worship. (Exodus 20:3; 1 Corinthians 10:14) The idea of the correct worship being given to Jehovah is so important that those who take part in idol worship are listed among those that won't "inherit God's Kingdom", which includes everlasting life. (1 Corinthians 6:9-10)

(New World Translation of the Holy Scriptures)