Why didn't I wake up after dying in a dream?
We still don't know much about dreams, and researching them is, at the most, a volatile kind of attempt to do science. It would be interesting what the statistics in the papers you read were based on, but the subject itself of those researches would be very unscientific because:


you cannot compare the "being dead in a dream" condition to any other condition that opposes it (not being dead in a dream doesn't count, if you look at it, as it means every other dream, and that is a lot of alternate conditions that might or might not wake you up equally)
you rely on "subjective" and hazy accounts of the event, hardly on a MRI scan or anything plausible (how would you choose specifically the subjects and situations where that specific type of dream occurs)?


...and so on.

To put it bluntly, the answer to your question is: because the science in those papers you read was, most probably, questionable if not downright bad.