Why didn't Mitch know about Morrie's illness?

From the text:

As for Morrie? Well, I thought about him now and then, the things he had taught me about “being human” and “relating to others,” but it was always in the distance, as if from another life. Over the years, I threw away any mail that came from Brandeis University, figuring they were only asking for money. So I did not know of Morrie’s illness.