Why is Canada better at graduating students and not saddling them with debt than the US?
More state support in Canada.
The US had no problems with student loan debt in the past. The cuts in state subsidy have caused tuitions to skyrocket.
Lack of finishing toward degree is related to student loans in the sense that high costs create problems for both.
I will say this however. Canadian students are more likely to stay at home for school since it is a way to keep costs down, and because the university system across the provinces is pretty uniform. Outside of McGill and U Toronto, the rest of the schools are of similar high quality.
There could be other reasons. For instance, Canadian schools could be better regulated so you don't have a lot of phony schools as we are seeing in the for-profit education world. If there are no for-profits in Canada (and I don't know that there are), then you will obviously see a higher graduation rate.