Because the successful trajectory (the survivor trajectory) is being analyzed on the assumption that if its attributes/variables can be reverse engineered, then the sequence can be reproduced by others (i.e., others will survive, too).
Some additional info if the topics interests you:
Survivor bias can be described as both a cognitive and motivational bias. First, it relies on the widespread conception amongst human beings that reproducing someone else's trajectory/decisions eventually leads to an equivalent outcome (cognitive error). Second, we all share a profound need to feel that we understand the world that surrounds us (motivational).
Think about the alternative: without this bias, we would be consciously aware that our success (or survival) cannot be singularly attributed to our effort or our ability to reproduce trusted strategies. This would likely push us towards feeling increasingly powerless in our ability to achieve our goals (or survive), and eventually drive us into a depressive state (the "whatever" mindset). At this precise moment I should bring religion to the discussion, but I will not because it will take me too much time.
Self-help literature almost exclusively operates by exploiting the survivor at two levels: authors often are themselves convinced that anyone who reproduces their recommendations will be similarly successful, and at the reader level, because without people telling us that we can succeed, there would be simply no hope.
Hope I answered your question :)