The SmartWatch 3 was Sony's first - and last - attempt at making an Android Wear device. It was a good watch at the time, and still is, but it won't be getting the long-awaited Android Wear 2.0 update. The SW3 was already missing from the list of supported watches, and now Sony has confirmed it.
The product page for the SmartWatch 3 on Sony's website has been updated with the statement, "Android Wear 2.0 and onwards are not supported," at the very bottom. Many hoped that the SW3 would receive Android Wear 2.0 because the watch already has NFC support (and there's even a petition with nearly 2K signatures), but it's definitely not happening now.
As for the reason, we can only speculate. It seems very likely that support was dropped because of the Broadcom BCM23550 processor (sometimes falsely reported as a Snapdragon 400) that the SW3 uses, whereas most Android Wear devices use Qualcomm or Intel chips. It's extremely likely the first-gen Moto 360 was dropped for the same reason - its OMAP 3 processor was already outdated when the watch was released.
- Xperia Blog