Comment by hnaccount_rng

I would bet the main forcing being insurance companies. There are only a handful of those and while they are powerful, they are also subject to (western) courts. And if (essentially) the US says: If you insure a ship using the wrong fuel you will cease existing, those ships won't be insured. And ships are too expensive to run without insurance (at least most of them). But.. pure speculation

Ports could be the other player, but how would you coordinate the ~all ports?