This theory conveniently ignores that streetcar companies also have lobbying power, and lobbying to stay the status quo is much more successful than lobbying for radical change.
It also doesn't help that a fair amount of streetcars were just land speculation plays (let's sell all this land that's newly accessible!). When they needed full replacement and renewal, they no longer had anything to finance it with.
Is it just switching costs being lower for streetcars? Or maybe the fact subways are underground means they don't battle in a zero sum pavement game with cars, so they were more immune to tides of public opinion and city administration?