I think some stories show up as earning $0 in a given month because they aren't getting enough visitors during that month to warrant even a few cents of profit, by whatever metric Medium has chosen to measure that monetization.

You're right that a "Make All Stories Eligible" button would be a helpful option for authors who want to go that route. I think that Medium requires individual opting-in of monetary metering, per each new story, so authors have more conscious control over whether they're wanting that story to generate money from the onset (or keep generating money). This might have to do with hypothetical copyright infringement claims (in the future) as well as Medium members who are only allowed to earn a limited amount of money per month due to disability status.