Developer Preview 3.0 adds some interesting features: support for fully round watch faces; on-watch support for a Play Store (important for Wear watches being used with iPhones, which don’t run the Play Store); and the ability to download apps over Bluetooth, WiFi or a cellular connection. Note that that last option would require a cellular radio, presumably using the emerging eSIM standard.
Why the delay? The developer’s note doesn’t say. But it’s worth noting that there are no new Android Wear watches from major manufacturers coming this holiday season. If there’s no new hardware in the immediate offing, it’s hard to justify the expense, pressure, and risk of a hurried quality control cycle.