Reason

Reason
 

¿Cómo se dice?

"What was their reason for why they didn't tell us before?"

"Cual era la razon para que no nos dijeran antes?

¿Cuál fue la razón que no nos dijeron antes?
or
¿Porqué no nos llamaron antes?

@Jelly: "Fue", not "era", unless this was a situation that lasted long in the past.
Otherwise, only check your spelling and the sentence will be alright.

@Poli: "¿Por qué no nos llamaron antes?" would rather ask why we weren't called before, but not why we weren't told something. "¿Por qué no nos (lo) dijeron antes?" would be an alternative. :)