Why is this verb Preterite and not Imperfect here?
"Sabíamos que Sebastian tenía un piso en Madrid. Pero nunca mencionó que tenía..."
Why 'mencionó' was used and not 'mencionaba'?
and why the second 'tenía' is Imperfect and not Preterite?
5 Answers
I think mencionó (Preterite) was used for it followed nunca.
Nunca = Never = Not Even Once!
'Once' was implied, so it was specific here, hence it's in Preterite.
You have basically answered your own question. "Not one time did he mention" (action started and finished in negative). However, without "nunca" it would still be preterito indicating that the action happened and finished one time only.
But if the mentioning happened habitually, you could use imperfect to indicate this.
I'll try again:
Sebastían tenía un piso en Madrid y durante esta época solía hablar de su piso con orgullo. Sebastían had a flat in Madrid and during this period he used to talk about it with pride. All in the imperfect, it was his habit to do so, over a period of time.
Sebastían tenía un gato en el piso y habló de este gato durante la fiesta de Nochevieja.
Sebastian had a cat in the flat ( uspeciafied time point, ongoing, imperfect) and during the New Years Eve party, he spoke about this cat. This was the only time he spoke about the cat and the point in time is specific. Preterite. ( he was a bore, actually).
Therefore, to my mind it means that even if he didn't speak about the cat it's in the preterite, although it could be in the imperfect if you want to stress that he never used to talk about the cat during that period. Do you see, the tense chosen changes the meaning slightly.
I hope this helps.
Beacuse he never mentioned it once.
It was most certainly not something he used to talk about..
One off, even if it didn't happen: preterite.
Try to think of the imperfect as a wavy line of flowing time and as the preterite as a dot on the line of time.
We knew - wavy line, there's no sudden moment where we knew otherwise we'd say 'we found out' so imperfect.

Sebastian had a flat.
Wavy line, there's no sudden moment there otherwise we'd say he got a flat.
But when someone mentions (or doesn't mention) something - that happens in a moment or in a set period of time and then it's over and done with so is more like a dot on the line of time, if you had to you could put a day and an hour or even a minute on it.
Some times those dots represent a huge amount of time rather than a little moment but whatever the amount of time it's always kind of encapsulated, finished at the time referred to, whereas the imperfect just refers to something when it was ongoing.

(not exactly the image I was looking for but hopefully you get the idea.
I think mencionó (Preterite) was used for it followed nunca.
Nunca = Never = Not Even Once!
'Once' was implied, so it was specific here, hence it's in Preterite.