[chasil]

As I understand it, both tetra ethyl lead and ethyl alcohol are anti-knock agents.

Lead was used because it was cheaper.

https://en.wikipedia.org/wiki/Antiknock_agent

[potato3732842]

Ethanol has other downsides regarding corrosion and reacting with various organic and organic-ish (read: rubber) components. Those were bigger problems back then because everything was carbureted and they didn't have modern plastics.

[consp]

Ethanol on its own is an anti knock agent. E85 is RON 105 for instance.