One reason student debt has skyrocketed is that public colleges and universities, which most students attend, were absolutely devastated by the recession. That led them to raise tuition prices. And although the financial picture looks better now than it did a few years ago, higher education institutions still haven't recovered.
The Center on Budget and Policy Priorities put together this interactive map showing how much per-student funding has dropped and how much tuition has increased since 2008:
When the recession hit, colleges faced a double blow. Higher education is countercyclical, so enrollments increased as the economy faltered. At the same time, the recession was laying waste to state budgets. The result is that states subsidize higher education less than they used to, and students pay more. (Hovering over states on the CBPP map makes clear that it's not a one-to-one relationship — in some states, such as California, tuition has increased by a much larger percentage than state funding has dropped.)
Now enrollments are dropping and funding is on the rise. But with three exceptions — including oil-rich North Dakota — the damage still hasn't been undone.