I think it's likely they wanted to reduce the number of writers earning money. When the pool of earners is smaller, each individual creator can earn more of the pot (theoretically), and also Medium keeps the scraps. So if you don't have thousands of writers earning less than $1/month, which I think represented a LOT of people, those dollars can be split between more active writers and Medium itself. The 100 follower requirement is seriously such a small hurdle, and it ultimately should make earning from Medium better/easier for those committed to contributing to the community rather than the passive masses.