The additives at the time didn’t work well; and simply removing the lead would lead to premature detonation, destroying engines. It wasn’t until the health effects were proven to be a big issue there was enough demand to change engine designs to be compatible with unleaded gas. That pressure was required as operating an additional set of incompatible fuel and engine types isn’t easy.

We still haven’t completely removed leaded gas from daily use. Namely small aircraft still use it for the same reasons cars used to use it.