Well, I'm sure you've heard about how much Pokémon Go has completely EXPLODED in popularity. The game hasn't just been successful, it's actually brought back full-blown Pokémania. Major news outlets are suddenly talking about Pokémon again like it's 1999, and everybody and their mother wants to go Catch 'Em All.
...Which stands in stark contrast to the comparatively little hype and excitement that Sun and Moon have gotten. Even discounting the fact that GF has been extremely conservative about revealing new info for these games, there just hasn't been a whole lot of enthusiasm for these games, even compared to what XY and ORAS got. For supposedly being the big "20th Anniversary" release, the promotion for Sun/Moon has been minimal, and even many dedicated, long-time fans seem to be "meh" on the games.
If the popularity of Pokémon Go is anything to go by, people want games that feel fresh and exciting. Pokémon Go took the basic idea first introduced in Red/Blue, traveling around to catch Pokémon, and gave it a complete makeover. Now, you can literally travel around in the real world (something fans have always fantasized about doing) to catch Pokémon. The game is both nostalgic and innovative at the same time.
On the other hand, I haven't gotten any indication that Sun/Moon will do anything that previous games haven't already done. The fact that the games will be on the 3DS already means that they probably won't be a huge graphical and mechanical leap from XY/ORAS. And, the novelty of new Pokémon and a new region wore off eons ago, especially when GF keeps on repeating the same tropes and archetypes over and over again. What is there really left to be excited for anymore?
GF should be very concerned about Pokémon Go, as the game could very well completely overshadow and destroy Sun and Moon. Seeing as Gen 6 already finished as the worst-selling generation ever, it would be bad news if Gen 7 suffered even worse sales. GF's lack of innovation may finally come back to bite them on the ass.