Historically, it is because UK government-backed student loans (for English students: no student fees in Scotland after all) are (a) low-interest, (b) only repayable on money earned above a particular salary threshold, and (c) automatically gets written off eventually after a specified period (was 25 years, now 30 years).
(a) is arguably not really true anymore (given some graduates are paying 5% interest), but (b) is even more true now than it was under the old (Plan 1) scheme because the salary threshold is now £27,295 (compared to the old one of £19,985).
(a) is arguably not really true anymore (given some graduates are paying 5% interest), but (b) is even more true now than it was under the old (Plan 1) scheme because the salary threshold is now £27,295 (compared to the old one of £19,985).