They didn't actually drop it per se. Sync 3 is based on completely different software, which was built from scratch by a different company. It's not a buildup of Sync 2.I wonder why they dropped it for Sync 3.
This isn't even the only feature of Sync 2 that's missing from Sync 3. Sync 3 is not capable of defining avoid areas for the navigation (one of the most basic features of a navigation system, if you ask me). You cannot hide the ambient temperature from the main screen (not without ForScan, anyway), though in Sync 2 you could. For a long time you couldn't see the ETA and the distance to destination on the navigation screen at the same time, which in Sync 2 you could - only in V3.4 did they finally address this issue (and they did it in a clumsy manner). And so on.