Ideally, the Society would like to revise the songbook every ten years. From the time revision begins to release is approximately 13 years (i.e., 1969 to 1982 for the last songbook). Translation new songbook lyrics is daunting. The reason the new songbook took longer than this is that many language groups did not have the New World Translation in their language and priority had to be given to Bible translation.
The many songs that were cut were judged to be inferior musically and lyrically. One of the manifestations of this is that they were hard to sing--even for professional singers.
For more detail on this, please see DNCall Unmasked, Parts I through V.
http://www.jehovahs-witness.net/jw/friends/180011/1/DNCall-Unmasked-Part-1-of-5 http://www.jehovahs-witness.net/jw/friends/180012/1/DNCall-Unmasked-Part-2-of-5 http://www.jehovahs-witness.net/jw/friends/180013/1/DNCall-Unmasked-Part-3-of-5 http://www.jehovahs-witness.net/jw/friends/180014/1/DNCall-Unmasked-Part-4-of-5 http://www.jehovahs-witness.net/jw/friends/180082/1/DNCall-Unmasked-Part-5-of-5