Of course it would be simpler and easier but who is going to pay for it?

Cambodia is a nation of around 15 million people and a large proportion of them are affected by iron deficiency. You are talking about trying to get skillets/ingots into the hands of hundreds of thousands of people, if not millions.

Even cutting the cost of each item by $1 reduces the overall cost to either the organization(s) running the program or the poor people of Cambodia by hundreds of thousands of dollars. The organizations running this program and the poor of Cambodia aren't sitting on an infinite pot of money.

The economic upsides from improved public health outcomes make it seem like a textbook case for foreign aid - it's easy enough to quantify the cost of the iron deficiency. Admittedly I'm thinking in autocratic terms rather than about the need for buy-in and all the other factors.

Sure, but the iron fish option is cheaper and just as effective. With the money they're saving but using ingots instead of whole skillets, they can solve other problems too.

The weight also plays a big role, especially since they're carried around a lot.

That's a point I didn't consider.