The practice of eating dog meat often originates from famine
Dog meat as a food source is usually an idea and action that comes into existence during times of famine. Dogs were both guardians for livestock and emergency food sources when food started to get scarce. While western culture dislikes the idea of it its still a popular food in eastern countries.
Many cuisines originate from times of famine and rationing. With the lack of ingredients at their disposal people had to start getting creative with making their food stretch or find new ways to make ends meet. Every culture has a few of these dishes in their recipe books. In America, lobsters are some of the most expensive seafood, but they were not always so popular. In the past lobster used to be considered poverty food and even jails weren't allowed to serve it more than a couple of times a week. With it being a coastal food, the invention of the railroad allowed it to be marketed as a unique food item. After a bit of time and some rising popularity, the poor man's meal was turned into the luxury dish people see it as today.  While in Western cultures there's a strong distaste towards eating what's considered a companion animal, with some Eastern cultures that's not the case.
As second world countries like China and Vietnam are evolving, so are their ideas on what is moral and immoral. As the country evolves, so does what is at the disposal of the people, not just trade but infrastructure and even the economy. When these things build up people gain more opportunities to benefit themselves and their families, which in turn gives the allowance of luxuries, such as more expensive food items, perhaps excluding dog meat from the menu.
[P1] Foods formed in times of famine often have sentimental value to those who have eaten it which have, in turn, popularized and increased the demand for the dish.