This was a very important issue for the government about eight years ago. They actually allocated a lot of money, around $15 million, to a national apprenticeship survey that was run in 2007-08. There had been past apprenticeship surveys, but they took this very seriously.
After that they commissioned a number of studies exactly on this issue. I was involved in that as a member of a technical committee. All sorts of possible reasons were asked about, in terms of whether there's an incentive to complete and so on. Often the wages of people who complete are not massively different from those who don't complete. Remember, you have a job if you're an apprentice and you can simply stay there.
There's quite a bit of literature on that issue. I don't think there's any one silver bullet that says this is the reason people are quitting. There's a wide variety of reasons why people are not finishing.