Over-dependency on a single industry, unwillingness to diversify, and an arrogant assumption that the same work could never be done elsewhere. Add in highways, commuting, and an infrastructure maintenance death spiral as domestic car production lost its major advantages. Rising crime added to the death spiral, as people who could moved out and the general deprivation of Detroit grew.

Many of the suburbs developed their own economies, which further encouraged people to move there instead of Detroit proper.

Lots of racism played its part in various ways. Lots of room to debate the precise degree, though the debate is probably not helpful at this point.

These days the high taxes (property and income) and auto insurance rates are big problems.

Only a few of these really apply to SF. Though the arrogance of assuming the good times will roll on forever seems accurate.