It is rare in the commercial world that a film concept totally relies on the acting skills of the cast. Most protagonists in this business are well trained models and rarely highly educated actors.
This film demanded a couple that would be able to show something that only really skillful actors are able to perform: true love.
On a tight budget, it seemed a promissing approach to cast real people really in love instead of middle class actors performing it.
An approach that delivered the desired result.