I think part of the IMO2020 compliance is that fines have actually been applied for ships that have broken previous similar regulations.

https://www.theguardian.com/world/2018/nov/26/cruise-ship-ca...

It turns out that the previous 2015 regulations around the USA and Canada were also largely followed, even offshore - this is despite there being little monitoring capability away from ports (I worked on this study).

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/201...

I am not an economist, but I suspect part of the compliance is a case of 'as long as everyone is forced to do it', we are okay with it as everyone can/has to raise prices.