The problem here is that these are not asset backed, at least in the collateral sense. A house can be foreclosed on, a car repossessed. You can't take away the education someone's gained, although I suppose you could force schools' registrars to not confirm the granting of a degree, but that could still be proven in a variety of ways.
The concern is that too many would immediately declare bankruptcy after finishing their education, wait out the 7? years it takes to for that to fall off their credit report and then get on with their lives. Since few new graduates need serious credit for anything beyond cars....
Well, I've been talking about the loans themselves, which by definition are not "assets": "Anything tangible or intangible that is capable of being owned or controlled to produce value and that is held to have positive economic value is considered an asset. Simply stated, assets represent ownership of value that can be converted into cash..." (http://en.wikipedia.org/wiki/Asset).
Therefore securities built on top of these loans, such as the pools you refer to in your link, are by definition not "asset-backed-securit[ies]". They are instead backed by e.g. the laws and regulations that make them non-dischargeable ... and I contend that's a major reason why they are non-dischargeable.