The RAE (similar to the Oxford Dictionary in English) includes ‘por qué’ as an adverbial phrase under the entry for ‘por’ meaning “for what reason, cause or motive”. ‘Porque’ is a conjunction that translates to “because”. And for fun, ‘porqué’ is a noun, similar to how in English you ask about “the why” or reason behind or for something. ‘El porqué’ would be “the answer” to the question ‘¿por qué?’