A combination of things that all pointed the same direction. As we continued to grow, physical mail became our biggest source of problems: letters lost in transit, address changes, limits on how much we can include physically in a letter, rising postage and printing costs that made it harder to keep the story quality high without raising prices. Digital delivery solves all of these at once with no lost mail, no postage constraints on what we can include, and better control of pricing.
This wasn't a decision we made lightly, and we know it changes something real about the experience for a lot of you. We tried to solve this in other ways first. Ultimately, we decided the story mattered more than the paper it arrived on.