At some point in time it was argued that manual allowed finer control of engine efficiency to automatic which simply shifted at certain speeds or rpms that weren’t always ideal. So properly driving a manual meant you saved gas.

I dare say in the decades since that argument began, automatic transmissions have gotten way better and reasonably as efficient as the average manual driver.

Also, when manuals were more generally common, they were generally cheaper than automatics. I don’t know if that’s true anymore, but I think the average person will have a hard time finding a manual new (consumer grade) vehicle in any given dealership in the US these days – you’d have to get it ordered.