A correlative meaning “why, for what reason” appearing in Demonstrative, Relative, and Correlative Stems (DRC) from 1948 (PE23/112), a combination of interrogative ᴹQ. ma and ᴹQ. -ro “reason”. It replaced (dative?) forms main, maina (PE23/112 note #148).
Neo-Quenya: For purposes of Neo-Quenya, I currently recommend maro as the Quenya interrogative for “why”. There is another word for “why” appearing in DRC of slightly different meaning ᴹQ. mámen “why, aiming at what”, but this uses the sense “aim” for ᴹQ. men- that was probably abandoned by the 1960s. Prior to the publication of PE23, I recommended using ᴺQ. manan “why = for what” created by Petri Tikka, a dative formation.
The correlative ᴹQ. iro appeared in Demonstrative, Relative, and Correlative Stems (DRC) from 1948 (PE23/111), a combination of the relative pronoun ᴹQ. i and ᴹQ. -ro “reason”.
Neo-Quenya: For purposes of Neo-Quenya I would update this to ᴺQ. yaro “that is the reason why” using the relative pronoun ya more commonly used in declined forms in later writings, such as Q. yassë “where [relative]”. For example: istan i saila atan yaro i osto alya (ná) “I know the wise man that is the reason why the city is prosperous”.