I would say many reasons.
First: today's current society is very body image focussed, so most people think about their bodies on a regular basis. It's hard to turn on the TV or go to the shops without seeing advertisements and products for all kinds of 'healthy lifestyle choices'. Add onto that the constant weight consciousness that seems to be abundant and you have people who are primarily concerned with how they look which comes down to health and fitness.
Second: Most people when looking to change their life is with their own health and fitness. It's the most visible area to start in and the one with the most answers, supposedly. Even Steve has said that if you are confused as to where to start your personal development, start with the physical.
Thirdly: I doubt it's the fear of death. Most people who fear death though act in a state of denial when it comes to their actions. They can't face their mortality, so they can't get healthy because they can't face that they are unhealthy. They can't even acknowledge what they are doing is going to kill them. Given this, I would say that most people trying to be health are doing so because they want to feel alive, so they so the things that will make them feel most vital. It's not so much a case of preventing death, but about actually living life to the fullest. It's impossible to do that without being healthy. It's those that don't fear death, but still want to live as long as they can. We're all going to die someday, but living longer and healthier is better than dying sooner and weaker.
Fourth: Weight management and fitness are such vast subjects with so much conflicting information that most people who try and lose weight, or exercise regularly fail. After multiple failures, they'll turn to seeking advice on forums for example.
Even reading many books don't help, they just give you more information to be confused about. Other subjects may be just as confusing, but they don't seem to be as prone to concrete failure.
Fifth: It could also be a hierarchy of needs thing too. Psychologically people seek physical wellbeing before they start going for the more abstract wellbeings higher on the list.