Por qué, por la que? Relative pronoun issue?

[Roxerz]

I was writing a paper last semester for my class making up a legend and I normally write "the reason why" as "la razón porque". My friend corrected me and she wrote "La razón por la que" which I never used it like that but it looks like relative pronouns which I never really know how to use. I asked her why and like usual, people tell me they grow up knowing what is correct because it is their first language but they cannot explain why it is used this way. So, why do I need to write it this way?

It translates to me, "The reason, for that,.." which gets the same point across. I know I shouldn't directly translate

excerpt from the story I had made up:

Toda la gente supo de Gloria y la consideraban la mujer más gorda en el país y todos comenzaron a llamarla “Gloria la Gordita”, ella vivió sola por el resto de su vida comiendo la misma cosa siempre y esa es la razón por la que hoy en día en México, llamamos a esa comida “Gordita”.