[xyzzyz]

Then the issue is to remember to take the pill. Much easier to forget than to forget to eat.

[cutemonster]

Also, can be hard to reach out to everyone and let them know that they need these pills. Many might not know what "magnesium" is?

It's simpler to add iodine to salt -- and maybe magnesium to the soil

[thaumasiotes]

Another issue is that the effect of a nutrient can sometimes differ based on what it is consumed alongside. Pills are not necessarily as effective as supplemented food.