Hanoi feels more novel to Americans than Ho Chi Minh, which feels like a fairly generic city. At least that's how I felt visiting the two.
Another thing you have to remember is that there are a lot of southern Vietnamese in the US, who brought their food with them. The average American has experienced more of south Vietnam than north without ever having visited.
Seeing the difference between a city that I perceived to be kind of emulating western culture (Ho Chi Minh), versus Hanoi, which has a culturally distinct feel about it, can reasonably lead a person to see a touristy city as a more cultural experience.
From an ameri-centric point of view, HCM is inundated with a kind of unpleasant or generic tourism (eat at these places, eat on a boat, go to this market, go to this tower, go to these museums, go to these "palaces," climb into vietnamese tunnels) compared to Hanoi where a lot of the tourism is related to both food and how beautiful the country is. It's kind of the difference between "this food is objectively good" and "this food is new and interesting."
