While vegetarianism and veganism are both suitable diets many people who are willing to swear off dog meat might not be so willing to swear off their favorite breakfast food or burger. This is due to meat being so hard to obtain in the past, giving it a high importance.
As the world progressed, so did peoples abilities to raise animals. With the industrialization of the raising and slaughtering of animals, people then had access to cheap meat. Lowered prices allowed for even poorer classes to serve meat to their families. This access and excess turned meat into a standard stable at the dinner table for people spanning cultures and countries.
Ease and accessibility has been the charm of today's food options from fast food to microwavable meals, many promoting tasty, juicy meat. While there are options for people to turn too, the real question is not why should they change their diets but why would they.