There are thousands of answers including, the nature of cancer, possibly didn't want to release how bad he was really doing, GOD said it was over, etc. Just what cancer does sometimes. What makes a healthy person who has ran thousands of miles, drop dead after a run? Who really knows?
It has been reported that he was having alot of problems swallowing, the effects of the chemo. The result was that he lost alot of weight.
With the type of attitude he had, and the faith that he displayed, he never would complain or show signs of being sick. RIP WWT.