Freaky claims and beliefs pique the curiosity of rational people. A flat-earther is a freak show that you've got to see to believe, which is why people spend time to go see it.
Is that you Mr. Barnum?
This is how the RCC sustained itself for centuries and continues to do so. All those pilgrimage sites had their freak attractions like a vile where the blood came back to life or the rock grotto where a magic lady appeared to children and then made a spring. These are freak show attractions of the first order.
All flat-earthers need to do to get a real foothold is work in a little religious freakiness. They need to find golden tablets and seer stones, discover a story about the first flat-earther who died for his beliefs, before ascending into the firmament. Once the IRS recognizes them as a religion they're good to go.
As goofy claims go a flat earth is pretty tame. No one is changing food into a zombie and then eating the zombie and drinking its blood. No one is wearing magic underpanties or funny hats or chastising the money changers. They need to teach how to find your flat zone and become one with your center. Where's their great teacher wearing a flat disc on his or her head? They need to seriously jazz things up, they're nowhere near approaching crazy enough to have a following.
Somewhere out there is the flattest spot on earth, the holy of holies, the sacred place where you can hear the flatness speaking. Honestly, these guys are amateurs and need a shot in the arm. They actually need a P.T. Barnum.