Because it is an outdated Act that needs to take account of modern requirements.
The UK Dept of Transport Regulations state:
Diesel: Diesel carried by private individuals is not regulated as regards how
much may be carried and how. Anyone contemplating carrying diesel in a
vehicle should bear in mind the potential for fire and the likelihood of spillage
from inadequate containment.
Petrol: Regulations made under the Petroleum (Consolidation) Act 1928 control
the quantities of petrol permitted to be kept in containers for private use. This
limit also applies to carriage. A maximum of two metal containers each up to 10
litres capacity, plus a maximum of two suitable and appropriately-marked plastic
containers each up to five litres capacity, can be kept in a motor vehicle.
This Act was brought in, in 1928 and I would like to see it amended. Engines today are much larger than in1928 and this law makes several trips to the filling station necessary in order to fill up my boat's tank. I would like to see the limit raised by at least double.