You need a lot more skill to work in an auto plant than you need for serving up fast food. 50 years ago, auto production wasn't nearly as automated as it is today, so the guys on the assembly lines needed to know how to do welding and other skilled trades. Robots do a lot more of that stuff today.
'Auto boys' are/were unskilled rubes.
Whites weren't holding up signs saying 'blacks not welcome'. They just left. It was 50 years ago, after all.
edit: They were, up to a point.