Car rego rip-off

Can anyone tell me why car registration in Australia is a flat fee? We are forever told user-pays is the economically rational and efficient way for things to be done. So why not fund the compulsory car registration/insurance via fuel taxes? Say the average car travels 15,000km p.a., using say 1500 litres of fuel, then 50c a litre would collect $600. People would pay according to how much they drive, instead of the existing system whereby low-km drivers subsidise high km drivers.