For the same reason most start ups fail by running out of money. They have no source of income but must keep spending more money to move things along.
What if issues arise in the toxicity studies, requiring further tests and thus delaying the trials once again? What if issues arise in the phase I trials? What if the stuff is nontoxic, but just doesn't work in humans? (We are not rats.) Again, the chart for success of NNVC requires that everything work. And that a company best known for missing important milestones actually gets things done on time.