Why has it been so difficult to discover a motive for the NIU shootings by Steven Kazmierczak?

I find it very disconcerting that there has been so little information relating to Steven Kazmierczak’s motive for his shooting at NIU. In the past, we have learned what caused this kind of unbelievable action, and, perhaps, have learned how to be aware of such possible mental illness. But this young man seems to have hidden his motives so well that there was very little evidence of what was to come. If there is no pre-evidence, how can anyone help?