Apparently they found recent grads with very large student debt load but commensurately high future earning potential had inaccurately low credit scores. Bypassing the credit agencies and doing their own scoring let them offer more competitive rates to that segment in particular, whose big loans probably carry a lucrative underwriting fee, not to mention bigger savings for every basis point you can shave off.
Interestingly they are underwriting as well as servicing ("Earnest will never pass you off to a Third-Party Servicer") which I see as a huge selling point. I wonder how hard it was to get the Federal approval for that?