This is mainly because the US gallon isn't a valid internationally recognized standard size, no one outside of the US can use it. A US gallon is significantly smaller than the UK gallon it's derived from, probably due to early american traders selling short measure
Originally Posted by Resoman
Litres are the International standard, scientists everywhere all use metric measures now. NASA used to make huge errors before they went metric !!!!
When Xtol manufacture ceased outside the US (in the UK etc) we were told it would only be available in US gallons, that didn't happen because no other country has measuring jugs/cylinders etc calibrated for US measures, so it went metric like 99.9% of the world.