Consumers can only choose from what options they're given, and if every option is leaded then that's what they'll buy. Also, if they're unaware of the negative effects of lead (or even unaware that their gasoline HAS lead) then there would be no incentive for them to choose what they're told is a worse product. This was before the internet, so it was difficult and time-consuming to do research on something like this. Gas companies said leaded gasoline was safe so everyone went with it.

The government's job here is to do that tedious research and ban/regulate anything that's hazardous to health and safety. The alternative would be a grassroots campaign by citizen experts who decide to go against the advice of what should be trusted sources, which is how you end up with stuff like the anti-vax movement.