Why was the subject of Nick and Laura's previous divorce never brought up?
The first point that might need to be made here is that Carver does not make entirely clear the past that Nick and Laura share. About the only indication that is clear about their past is that, like everyone that evening, they are "from different places." Nick and Laura are not the primary focus of the story. Carver sets them up as if they are like us as the reader. We, like they, are witnessing the carnage of the relationship between Mel and Terri. Whether Nick and Laura were divorced or what they were like in the past is secondary to this. It is for this reason that there is minimal description of Nick and Laura.
The focus of the piece, the entire driving force behind what the characters actually speak of when they speak of love, is on Mel and Terri. Their differing conceptions of love in terms of their past and present, as well as how they view love in the future is spoken. Nick and Laura as well as their own past is almost a distant second to witnessing what is present between Mel and Terri. Their past and whether or not they are divorced has little bearing on the development of the discussion between Mel and Terri, reason enough for Carver to not mention it or discuss it. In the end, the question that the reader is left to address is whether Nick and Laura will prove Terri right in that they are "in the honeymoon." This becomes the basis of their future discussion of what they will talk about when they talk of love.