Likely because you're viewing it more than 45 years after it was drawn. Our ideas about information visualization - and the tools available for creating or - were a lot less sophisticated back then.

I actually think the graph is great, once explained. It gives a good visual summary of both things it's trying to convey (the overall point, and the different speed at which incidents can develop).

Also interesting to think of this writ large. How many companies do you know of that are hanging out in the middle section without realizing how close they are to a point of no return?

A lot of graphs don't make any sense unless you view them as part of the talk they're presented in.

I see a lot of bureaucratic NHS slides that are meaningless until someone talks you through them.