Since there has been world-wide peace in the world for the last few centuries, wars are usually only localized conflicts, people do not see the need to be able to fight and they are more sedentary, so they are now seeking ways to “stay in shape.” Since people are drawn to the more exotic, they tend to choose things that are different, from the ordinary. Some people come to martial arts for self-defense, but studies have should that most people come to “get into shape” while socializing. People in marital arts classes interact while they train. In aerobic classes, you train continuously without any interaction and, after class; you are so exhausted you just go home. There is no camaraderie as there is in martial arts classes.
If you want to learn a martial art and increase your fitness level as a fringe benefit, then Taekwondo is a good choice. A person who participates in a TaeBo or Aerobic Kickboxing class for an hour a day will be more fit than they would if they participated in a Taekwondo class for an hour a day, however, a person who participates in Taekwondo for an hour a day will be more fit than they would if they bowled or played golf for an hour a day. In a TaeBo class, you are not there to learn to perform perfect techniques; you are there to perform continuous aerobic movements. It Taekwondo class, your goal is to attain perfection of mind, spirit, and body and to become a good fighter and better person. In Taekwondo, you mostly train in an anaerobic (without oxygen) manner. You train for bursts of maximum output of energy, not continuous exertion. A person can easily hold his or her breath for 60 seconds. Think about how many kicks and punches you can execute in 60 seconds. It is like 100 meter sprinters; they train for maximum output for 10 seconds. They do not need to breath during those 10 seconds.
For Taekwondo purposes, running sets of sprints for an hour with short breaks between is more useful than running continuously for an hour, since fights, real or sparring, do not require the a steady output of energy but rather short bursts of energy during clashes punctuated with long periods of maneuvering. Therefore, aerobic fitness is not required.