The imperfect tense is used because it refers to a longer lasting state as kind of a background description.
Another example:
Nous avions froid et faim à cette époque-là.
We were cold and hungry those days.
Hier nous avons eu faim donc on est rentrés.
Yesterday we were hungry, so we returned.
We were cold and hungry those days.
Hier nous avons eu faim donc on est rentrés.
Yesterday we were hungry, so we returned.
In the first example the description is about a longer lasting state and describes sets the scene for maybe a further statement whatever might have happened then.
In the second example it is about getting hungry and then returning, i.e., two occurences which both describe what HAPPENED (i.e., an action) and not what WAS (i.e., a state of being).
I hope that helps,
-- Chris (not a native speaker).
