Here are some thoughts of mine, in no particular order:
1. I have noticed that an awful lot of bisexual poly people only want ONE partner of each gender. It's quite possible that J. was one of those, was more interested in the relationship with your wife, and went along with things with you so she could make things happen with her. And then, when your wife wasn't her ideal sex partner, she moved on to someone else.
2. J. should not have told you she loved you if she wasn't feeling romantic love towards you. That was wrong, and cruel.
3. There may be things you don't know about the relationship between D. and J. One of them may have gotten upset at the emotional pace of the relationship in one of the pairings, and wanted to back off. One of them may be having sexual dysfunction. Their marriage could be rocky. J. could be having a sexual identity crisis. It's impossible to say what might really be going on. My husband and I dated another couple for a few months ourselves, believing their relationship to be solid, and then their marriage imploded in their next quad experience with some friends of ours, and our friends' relationship was a casualty too.
4. Quads are one of the trickiest poly relationship dynamics because so many things need to work. I would love to have a successful one myself, but I think it's a long shot.
5. Getting put on the shelf just sucks, and I'm sorry that happened to you.