Because a QTreeWidget behaves more like a List than QTableWidget. For example you do not have to care about hiding the gridlines (which you normally do not want to see in a list). Also in a tree you have one Item per Row, in a QTableWidget there's one Item per Cell... but in a usual list, you work row-based.
As you said, a table can of course be configured to look like a list, but a tree can be configured to look like a list with less coding and behaves more like it is expected (at least by me) from a list...