"How do you get people to ____" is a fundamental question in life with many approaches to answering it, ie marketing, public relations, missionary studies, theater, etc.
And, yes, no shit. How to get other people to do something is a fundamental question in life and it just so happens that in life, public health is about getting people to do stuff that's good for their and society's health.
From what I pointed out below, this salt processing seems to have underappreciated trade offs. But most importantly, the overarching picture of nutrient availability, not just for iodine, seems to have a bigger challenge at hand worth solving than simply using salt as a vehicle for one - which spirulina would help with in terms of protein and mercury / biotoxin removal, for example.
The point is that you don't go looking for a new thing for people to consume to get them the nutrients they need, you fortify what they already eat.
And, sure, there may have been some other equally good alternatives, but analysis paralysis helps no one. So, salt.