+2
To answer the OP, my immediate thoughts are because they were celebrities, not just gangsters. There'd be pics of them in the papers with film stars, politicians and pretty girls, at casino's and nightclubs (especially once we get into the 'swinging London' era) and were interviewed on TV. Everyone knew what they did and who they were, but it couldn't be proven, so they had the Robin Hood quality to them too, which the working class everywhere have always loved. It's the attitude of 'yes they're a bit naughty, but they only hurt their own and they've never done me any harm, so if they can get away with it, good luck to them' It's completely untrue, of course, but that's why myths are myths and why they last and, because they're myths, they have value and money can be made from them - hence the films.
As for the young and those not from the UK, the films are violent and their lives give grounds to show that violence. It allows filmmakers to use exploitation levels of gratuitousness while having some justifcation to back up their use.
I don't know if that's why, but it's what immediately came to mind.
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5-time MoFo Award winner.