My guess is that some psychological factors make them impossible to use. Maybe some combination of that bias where most humans won't wait some time for a large

My guess is that some psychological factors make them impossible to use. Maybe some combination of that bias where most humans won't wait some time for a larger reward later, and the amount of effort it takes to keep track mentally of where you're making a micropayment, and where you're not. I would guess that the closest analogy would be metered long-distance telephone time vs flat-rate. I bet that a significant portion of those going flat rate actually paid more for it than when they were metered.