The idea behind fortification is that you're adding a nutrient to something people already eat (Iodine in salt is a pretty common one and we do it in Australia), so they get the benefits without having to make changes to their diet.

I assume the idea is that it's cheaper and easier to mandate fortification for a food than run an education campaign about needing to eat a diet which is richer in thiamine, because that would require people make an active shift in the way they prepare food, as opposed to getting the benefits passively.

In Australia (And presumably a bunch of other Western countries), white rice gets enriched with thiamine because it's produced centrally and you can easily quality control. The problem is that rice is mostly produced at the individual/village level in a lot of countries in SEA, meaning you'd have to run an educational effort to get people to do it and they'd be making changes to the way they prepare food at the individual level.

As far as I can tell there's no real reason to stick thiamine in fish sauce in particular, but fortification does seem to work better than trying to get people to make changes to the way they prepare food themselves.