The food in the article and your food seem quite involved and niche (Thai, vegan, Nordic). Does a cart like Potato Champion do better because the food is easier to produce and everybody likes French fries?
Even Potato Champion serves some niche foods though, with Poutine and their fancy ketchups/aiolis. I think you're probably right, though, in that Potato Champion seems to have hit the sweet spot between niche/broad appeal in their market.
I don't know much about the business, but it seems like all the food trucks in my area in Colorado are niche foods. Occasionally there are events when food trucks show up at local parks, and there is a wide variety. There is usually several types of ethnic food, a truck or two with food sourced from a local organic farm, and a truck from a local brewery. It seems like food trucks appeal to the type of person who shops at Whole Foods or Trader Joes. In fact I think I remember seeing a truck serving food from peru or brazil, when Mexican food would be less niche. Because Colorado has a bunch of immigrants from Central America and has a number of Mexican restaurants.
It also probably helps that potatoes are cheap.