Witnesses do not promote an unhealthy lifestyle
BUT
they DO NOT promote healthy lifestyles. Without motivation to live a healthy life and with all the marketing propaganda targeting weak willed people, pushing them to eat the fast JUNK FOOD, the easy way is to just give in and put your hope in the New System. If elders starting excercising and getting in shape, which would be a good example to the congregation, there would be far more negative comments than positive.
Imagine an elder starting to get in shape and show some definition, muscles starting to pop out of his short sleeve shirt, walking with a spring in his step that leaves everyone else in the dust. How many congregations could or would tolerate such conduct without murmurings and talk of worldly influence, whereas all the poor sap wants to do is feel better about himself and get rid of his aches and pains.