In a nutshell, as a generic reasoning, say that a few years ago the student loan was about US$ 100,000 and once you got the degree you had - on average - a higher pay of around US$ 10,000 per year (net).

You could repay it in 10 years time while living at the same level of your non-graduated friends.

Now say that the student loan averages US$ 150,000 and - still on average - the increase in annual wages is US$ 7,500 (net).

So you can either:

1) live as before BUT repay it in 20 years instead.

2) live at a 7,500 US$/year lower level than your friends and still repay it in 10 years.

3) live at a 7,500 US$/year (or 10,000 or 15,000) higher level and never repay it.

I suspect that besides the lessened "value" of the university degree on the labour market, there is also an increased number of people that choose option #3 over option #2 or #1 (whether out of need or because of other reasons doesn't really matter).