Why didn't I wake up after dying in a dream? So I found a lot of articles about the facts why death in dream wakes us up, like that our brain doesn't know what happens after death, or because of the adrenaline and so on... I had two dreams in my life where I didn't wake up after being killed. I only remember one of them where I have been chased by a woman (the killer) and I was running, I was so nervous and scared and then she caught me and took me somewhere with some indians keeping telling me "It is going to be ok/cool/alright" and I was tied, I noticed the axe she was holding, it was so scary, and then she cut off my head. No awakening. I saw indians taking my body somewhere else, the woman came to me and I told her "It is ok, as you said" and I was so satisfied, filled with a feeling like Why was I scared?/It is absolutely alright... and the dream continued, but nothing special happened.
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.