I think it is simply because a large number of adults in the US (as well as a lot of other countries) is struggling with this, at about 39% obesity rate and 71% overweight rate [0]. For comparison, only 1.5% of adults in the US are underweight [1].
While I agree that there is way more to "being healthy" than just not being overweight/underweight, I think it isn't a bad idea to approach this in the priority order. Solving the issue of being overweight is much simpler and much more impactful to the overall public health than trying to go after more rare and difficult problems.
I mean, you have two markets and one is about 47 times the size of the other.
If your goal is to minimize the total number of health problems in your country, it would make sense to throw more efforts at a problem that affects 47 times more people, given that everything else about the problems is about equal. We are not comparing cancer to heart disease here. We are comparing two problems that are two sides of the same coin.
While I agree that there is way more to "being healthy" than just not being overweight/underweight, I think it isn't a bad idea to approach this in the priority order. Solving the issue of being overweight is much simpler and much more impactful to the overall public health than trying to go after more rare and difficult problems.
0. https://www.cdc.gov/nchs/fastats/obesity-overweight.htm
1. https://www.cdc.gov/nchs/data/hestat/underweight_adult_15_16...