Because students generally are poor and so don't have much to lose by declaring bankruptcy. They shrug off massive debts are left with an (ostensibly) valuable degree. Lenders are unable to secure the loan against the borrower's assets, as would generally be the case for a home, auto, or business loan. It's possible that if student loans were dischargeable in bankruptcy that, absent a government guarantee, the lender would need to charge a high enough interest rate where the student loan market simply wouldn't function. And a government guarantee combined with student loan dischargeability in bankruptcy would leave US taxpayers holding the bag.