Why were some preachers against rock music?

Answer 1

Undoubtedly by some people. Beatles music, which is extremely mild by today's standards, was protested, boycotted and even burned and steamrolled publically in giant heaps. Some music, it should be mentioned, is heretical - including a few classic beatles tunes. The song Imagine by John Lennon invisions a united future world without poverty, hunger, war, "and no religion too..."

Answer 2

They're typically against secular music in general, and they base this on an interpretation of The Bible that music must be directed to the praise of God, and be driven by melody (as opposed to Rock and other contemporary forms of music, which are beat driven). Additionally, Rock Music tended to have a much more "decadent" message than others popular forms of music at the time, which furthered their opposition to it.

Agreeing totally with the previous post I'd like to add that Heavy Metal in particular sometimes uses chord or note changes that are six semi tones apart eg the song Black Sabbath by Black Sabbath. I believe that from many years back (possibly in the monastries) that this chord/note change should never be used in religious music. Whether this has anything to do with some modern preachers disliking the music I have no idea.