RUCs existed well before EVs were a thing. They apply to diesel vehicles (as unlike petrol, diesel doesn't have a road tax on it). EVs were temporarily exempted

RUCs existed well before EVs were a thing. They apply to diesel vehicles (as unlike petrol, diesel doesn't have a road tax on it). EVs were temporarily exempted from RUCs (partly as an incentive, and partly because RUCs on light vehicles are a lot higher than what an equivalent fuel-efficient vehicle will pay in road tax through petrol usage). The current government removed this exemption (without adjusting the RUC rates to a sensible level for EVs), causing BEVs to pay significantly more road tax than an equivalent fuel-efficient non-EV does and removing their advantage of lower running costs (which has massively hurt the EV market). They "justified" this with their vague plans to move petrol vehicles to RUCs at some point in the future.