The Mac app store is a terrible platform for distributing games. People do not look there for them, and they have a bad reputation of breaking or being left unmaintained. It has awful discoverability and is filled with trash. Customers also prefer to get them on Steam anyway (or on consoles). This is especially true if a game has online support. Most other distribution platforms also give customers access to the game on any platform, rather than locking them to one. It doesn't really offer the consumer anything, and for a developer who will have to target Windows and Steam anyway to have any chance of making money, it isn't really that enticing, even with a larger revenue share.