Because they aren't and never will be "micro". Every single media I saw trying this eventually comes to the multiple dollars, or tens of dollars per single purchase of a single digital item (post, video, recording etc). That's not sustainable for majority of people, and of course fails. Unless a specific type of people become interested in it, commonly known as "whales", who can sustain a lot of expensive single purchases. And it's a reinforcing loop - since more money comes from a few "money is no issue" crowd, creators are increasing prices to extract more money from this crowd, more people are filtered out and more whales are left, the prices increase again and so on.