What does Por que no mija mean?

Answer 1

Mija is not a real Spanish word, but is a slang contraction of "Mi hija".

The sentence should really be "?Por qué no, mi hija?" which would be translated as "Why not, my daughter?"

Answer 2

"¿Por qué no mija?" in Spanish translates to "Why not, sweetie?" This phrase is commonly used to ask why something has not been done or why a certain action has not been taken in a friendly or endearing manner.