I think the problem is simply hard. Micro-payments imply more infrastructure costs (more transactions) but lower revenue (because the payment itself is tiny, so

I think the problem is simply hard. Micro-payments imply more infrastructure costs (more transactions) but lower revenue (because the payment itself is tiny, so the fee has to be tiny as well). But look at Bitcoin's Lightning Network, it may very well become a valid micro-payment system. As far as I know it's biggest scaling roadblock is getting efficient routing.