Sure, the purpose of those taxes are completely different though. Emissions taxation obviously makes sense to apply per unit of fuel - if you buy 1L of petrol

Sure, the purpose of those taxes are completely different though. Emissions taxation obviously makes sense to apply per unit of fuel - if you buy 1L of petrol you're almost certainly going to burn it, and the CO2 produced is going to be pretty much identical regardless of which vehicle it's used in or whether it's used in a generator or whatever. Road wear is more indirect and really just sort of works out on balance (e.g. some pay more than they should, some less). With an EV only the road wear is relevant. If you're inspecting the car, it makes no sense to use kWh (a proxy for road use) when you can just use mileage (the actual road usage).