Not a definite answer and a bit of guesswork here. My thought is that just saying "whys" one can think of them in separate events, as we see mostly to describe Japanese techniques, for example, The 5S, The 3 5 whys. In English we have the "WH" questions - who, why, where, when, what, ...etc.
But back to the crux, somehow the 13 whys would be the set of events which led to Hannah Baker suicide. According to the accounts that were in thirteen letters sent to 13 people (that happens to be her friends -cough cough..., colleagues).
So, thirteen people were to blame for the tragic event, according to the author of the letters.
Ah, in this case it was better to say "13 reasons" because they were 13 different "whys", thus the concept here is different of the one that have to do with "techniques/system".
Plus, the "whys" are separate but interwoven as well.https://pt.wikipedia.org/wiki/Thirteen_Reasons_Why
Our "porquê" (noun) is usually preceded by "that" or "here" in English: "that´s why.
"/"here´s why" (eis o porquê)
One tip about learning:
Try to compare our grammar with that of English, but try to tell one of the other. I mean, see the similarities but beware the misleading that comparisons may create.
There are some important differences between them and the reasoning that works to one doesn´t necessarily works to the other!