I'm not too sure, for me I think it was down to crap teachers full stop.
Up to the age when the lessons stopped I was open to any type of music or instrument. I enjoyed the lessons where we heard classical music and really enjoyed it when a military band came to the school (the trombonist had a huge pointed nose and a wicked sense of humour). But having a repertoire of Peter and the Wolf, Carnival of the Animals, Young Person's Guide to the Orchestra and Sparky the flipping Piano that just get used as class silencers and lessons that just cover the same basics over and again.
Imagine trying to learn English with readings from the same four novels, lessons that never get past basic grammar and only ever being asked to read aloud from the same Janet and John books.
<Edit>
I do know what good music teaching is like, because I took singing lessons. The first lesson explored my voice and my aspirations, and over 10 or 12 weeks explored my range, made me learn to phrase things, made me aware of and control my breathing and revealed that I actually had excellent relative pitch. It was all focused on 'where are you now, where do you want to go and what is holding you back'. The only thing was I had to fake the sight-reading by memorising the melodies, easy as they were always played though first 🙂