The main reason is that gasoline cars allow for a breakdown of the free market. You can pollute for free, making them artificially cheap, a subsidy on a huge scale.
A trash collector can’t throw the trash he collects in your yard to save $, Jiffy Lube can’t pour used oil in a lake to save $, you bring your unused paint to a recycling center.
The trash collector pays to dump at the landfill. Jiffy Lube responsibly handles the oil, at a cost more than pouring it in a gutter.
But gas cars, they do none of this, they take the cost associated with their behavior and poof…make it everyone else’s problem and expense. Like LeBron James blowing his talcum powder around.
There is no line item on the receipt for a gas line purchase to address this, the asthma attacks, the ozone creation, the acid rain, the climate gases. If there were, has would be much, much more expensive.
Another way to eliminate this subsidy would be for gas cars to bottle their exhaust and dispose of it properly. Very expensive and probably not technically viable.
If gas cars were to take either of these steps, eliminate their subsidized behavior, they would become prohibitively expensive.
To specifically answer your question: because air pollution is the archetypal subsidy and subsidies are bad economically and, in general, morally.