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.

0. https://www.cdc.gov/nchs/fastats/obesity-overweight.htm

1. https://www.cdc.gov/nchs/data/hestat/underweight_adult_15_16...