Habits, personal taste, etc.. Also, I was a little tongue-in-cheek and didn't present her POV, which does exist. So, to be fair -

The scenario I described, when she needs to roll both bags---and no cart is available---does happen but not too often. More commonly, when we get to our "main" destination, there are a few 1-2 day side-trips with carry-ons, leaving the big bags home. For these trips, I still prefer my shoulder bag, but she prefers her rolling one both in convenience and appearance.

In the end, all said and done, the reality is that she counts on me being around and is not optimizing as a single traveler. For example, one of my big deals about my shoulder bag is packing space and compartments, but then she does feel free to use those too as I carry the bag. So telling her that a shoulder bag would have been better for her if she were to travel alone is like telling someone who is enjoying their boat in Lake Superior that if they go to Sahara the boat would be useless and they'd need a camel instead.