I speculate that it's the ports which make the rules and they're not bothered by having to charge more for fuel, as ships are wholly dependent on that. Meanwhile local authorities can put pressure on ports to not provide certain types of fuel.

When maritime shipping quintupled in price during the pandemic it wasn't because ship operators suddenly figured they could fleece people like that - it was the ports' logistics which were all out of whack.

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?