I don't know how long you have watched The Voice, but there have been other instances where finalists have chosen to bring back only one person with whom to sing. The example that immediately springs to mind happened during season 2, when Tony Lucca brought back Jordis Unga (and ONLY Jordis Unga) to sing Fleetwood Mac's "Go Your Own Way". I can't recall off the top of my head any other examples, but I think it has happened more than these 2 instances.
The reasons may vary as to why some finalists may do this, but depending on the artists and the song, sometimes a duet can be more impactful than a group performance. This being said, I do like to see as many contestants "brought back" as possible, and although she was not my favorite, I would like to have seen Hannah in one of these performances. I enjoyed the energy she brought to the stage.