We didn't "switch" to unleaded gas in the 1970s. We added unleaded, required gas stations to offer it, and vehicle manufacturers were prohibited from making new cars that required it.
Leaded gas was still being offered at some stations well into the 1990s.
Fuck, Germany introduced unleaded fuel only in 1984 and still offered leaded fuel for a long time.
It was required because of the introduction of catalytic converters, which would get damaged by lead in the fuel.
When I was at Bundeswehr I was in hospital for some time and got some thorough diagnosis. They asked me if I worked with lead in the past (no), so I seem to have at least some lead levels in my body.
As gas stations have enough room for more expensive "ultimate" versions of fuel, I don't think that's the reason. You can frequently get two kinds of Diesel and three kinds of Gas at the pump.
O. t. o. h..maybe the whole "premium fuel" hype resulted from stations having additional tanks after leaded fuel was phased out.