Thinking about this a bit, it kind of makes sense that when gerber introduced more expensive citizens to be mined (young citizen and students), he also nerfed the citizen mining by using war tax adjustments.
Your 500 popularity cities mine as before with no war tax (50 popularity per tick, or 10% every 2hrs at 500 popularity). But with the introduction of the war tax slider, its become much less likely we have days without war tax as current administrations have opted for a 'moderate level' of war tax.
So this effectively increased the popularity required to mine effectively, but the young citizens and students sell at prices much above the homeless and immigrant cards (which we still get as well).