Well, probably because in English we don't really have an equivalent of the Spanish imperfect (trabajaba) to express an ongoing event in the past. So if we want to make it clear that some event was ongoing in the past (trabajaba / estaba trabajando) when a specific event occurred (conocí), we typicaly use the past progressive.
EDIT: And a question for you, if you don't mind. In this particular sentence in Spanish, is there much difference between "trabajaba" and "estaba trabajando"? (I don't think there is.)