My personal theories:
The old pink 1966 songbook I grew up with had a lot of songs that sounded a lot like traditional hymns, in fact I think some of the tunes may have been the same as hymns with different words. The feel produced was a familiar and comfortable one to the American protestant (non-Catholic) heartland from which the WT movement got its roots. Since then a few things in the landscape have changed. Overseas growth and outreach to non-protestant backgrounds meant that the songs didn't seem familiar to potential converts. Some of the songs and/or tunes were written by non-JW or worse yet, ex-JWs (anathema to JWs). There may have been copyright issues for some of the music. Also, some of the old words, while seemingly innocuous to some, probably came across as disturbing in other areas of the world (her breasts are like towers; bees that were molested; etc).
The various songs and songbooks introduced starting in the '80's I think have been their attempt to modernize and get away from the US hymn-feel. Other churches have done the same (ie Christian rock). I don't think the WT had been entirely successful in their attempts, but I think that is where they are going. (I personally don't care for most of the new tunes, although a couple of them are not too bad)