Probably so you can't game the system.
A non-profit marketplace for this built similar to Kiva, LendingClub, etc. increases transparency dramatically.
By gaming the system I meant that someone might go into debt, don't pay, wait for them to offer it up on the secondary market and then buy back the debt for pennies on the dollar effectively forgiving their own debt. They might need to have someone else buy the debt for them but it's essentially the same process.