The simple answer to that question is that it depends. Let me explain...
Many people do cardio with the goal in mind to loose fat. And a big part of the ones who want to drop in weight are obese and out of shape. For those it is best to go for a walk. If you do Interval training when obese and out of shape you would most likely injure yourself due to the extra pounds you are carrying and the simple fact that your body cant handle the sudden stress. Yes, HIIT burn more fat in a shorter amount of time, but you are going to gain it very quickly again if you injure yourself.
For athletes on the other hand High Intensity Interval Training is a part of the training schedule. Athletes have to do HIIT to improve their performance (not so much to loose fat) and their bodies are used to exhaustion and other stress factors. Injury is also not that likely.
Walking: For people who are overweight and out of shape to gradually shape up and loose some pounds.
HIIT: For athletes who want to increase performance.
