I wouldn't call Freesync "great", more like serviceable.. This is judging based on user reviews and feedback I've seen since it launched. I've also seen some of the top review channels put it head to head with G-Sync and claim G-Sync performs better.
People are opting for Nvidia because they are better cards, and Nvidia has better supported drivers. Most games are created with Nvidia optimization in mind, it's rare you see a developer opt to feature AMD's gaming brand for their game.
Nvidia is the trendsetter, constantly creating new features (that actually stick around) and graphics technologies that enhance the games look.. AMD has been playing catch up to Nvidia for as long as I've been into PC gaming, and their sole selling point, although not one I give a about, is price to performance ratios. Cards like my 6 GB 1060, especially when found at a great price like I did, really on that theory though.