Wiki says it was more because of the private right away used, not necessarily its steepness:
> While many streetcar lines were converted to bus lines after World War II, the J Church avoided this due to the private right-of-way it uses to climb the steepest grades on Church Street, between 18th Street and 22nd Street.[9]
> Ironically, this steep grade saved the J-Church line from being replaced by buses. After World War II, San Francisco began replacing the streetcar lines with buses, but the Church Street hill proved too steep.
> While many streetcar lines were converted to bus lines after World War II, the J Church avoided this due to the private right-of-way it uses to climb the steepest grades on Church Street, between 18th Street and 22nd Street.[9]
https://en.wikipedia.org/wiki/J_Church