Voltage is originally not created using in the KMS system with kilo, meter, and second but a unit in the CGS system that has the base in centimeters, gram, and second. That were the customary units in science when the field was created.

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It is when International Electrical Congress (IEC) defined the unit in 1881 it was scaled up for the voltage in the MKS system, International Electrical Congress become International Electrotechnical Commission in 1906 still called IEC, and is still a standard organization for that field.

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So it is units that were very quickly defined within the international agreement before any large-scale adaptation.

This also shows there is no metric system, there are many metric systems. MTS(meter-tonne-second) was used industrially in France and the soviet union in the first half of this century.

What is commonly used today is the SI system which is A metric system, not THE metric system.

The CGS system is still used in for example magnetism. The usage have decreased with MKS standards in 1940 and the SI standards in the 1960s. It has been used longer in theoretical sciences compared to practical engineering.