I think one of the major factors is that you can have something bad for you but not suffer any immediate ill effects, particularly if you don't monitor your blood sugars. If I had a chocolate biscuit now, I'd go up, but I'd either correct or I'd come down with time and literally nothing would happen to me. It's not a huge step to go from there to 'well, I'll have a couple - that can't hurt' and then to 'look, it's only a kilo of chocolate'. Added to that, the complicated nature of what you can and can't have, the fact that people vary, the sheer depressing nature of permanent lifestyle changes for a chronic disease and frankly I'm impressed so many people manage it so well. If you vomited or had explosive diarrhoea everytime your blood sugars went over 13, I guess there would be a lot of very well controlled diabetics.
Also, I'm sure not everyone feels the same, but for me caramel shortbread tastes much nicer than okra or prunes.
I watched a documentary about young diabetics in Southampton. The consultant said that he was asking young people to fingerprick themselves, inject, avoid drinking too much, stop smoking, take more exercise, live a restricted lifestyle that their mates didn't have to, all so that their 35-40 year old self didn't die of a heart attack. It is, as he put it, a big ask.