The run up to commitment story has, in my opinion, more possibilities for story development than a post-commitment story. Character A might choose X or Y or Z - or none of them. Or A might choose one of them, only for them to refuse. As a writer, there are more places to take the story telling, more possible twists and turns.
If A is married to B, then essentially the story can only go in one of two directions: by the end, A stays with B, or A leaves. What happens en route will vary, but essentially these are the only 2 choices. If A leaves B at the very beginning of the story, there are still two options: the story ends with A emotionally leaving B, or getting back together.
If A leaves B at the beginning of the story and has a high old time choosing who they're going to go with next out of X, Y or Z then we're back into the run up to commitment story, not the commitment story.
Basically, the options for a run up to commitment story are Open, when the options for the commitment story are Closed.
I've written two commitment stories and three run-up to commitment stories (with a fourth on the way), and the run-up stories are frankly much more interesting for me to write. You can go anywhere! Your characters can do anything - and anyone. Whereas, once your character is married, if they want out there is a known legal process to follow. Even if the character simply walks out, as in Ladder of Years by Anne Tyler, the story is still at heart about whether the marriage holds or fails at the end.
That's not to say that stories with Closed options are essentially less interesting than stories with Open options. But because the reader knows where a Closed option is leading to, I think the writer has to work much harder in keeping the reader's interest going.