It's a very long story, spelled out in their Kickstarter updates and comments. Briefly: they made the prototypes on a 3d printer. A mold was made based on that prototype, but the manufactured versions did not fit as expected. Ben and co. flailed through various iterations of materials, cooling time, etc trying to get what they thought was a good fit. Each iteration took time and cost money. Finally they got some expert advice that seems to have guided them to the correct material and manufacturing process.