We got a 1.9 millions number a few months after the worldwide launch, but later there was a chart for Heroes' launch that had Fates only around 1.2 millions, IIRC, with Awakening above it with 1.6 millions. It's not clear what that means, but most people seem to assume that the earlier and higher number was counting the Conquest/Birthright DLC paths, rather than just the base games. If that speculation is right though, it seems like Fates saw a significant drop in international numbers coming from Awakening, considering how it sold more than Awakening in Japan itself.
That makes more sense.
1.9 1.84 always seemed too big of a launch for a franchise like FE, specially when you consider Fates had considerably less buzz than Awakening back then. It never even reached OT2.
It's true that Awakening came at a very fortunate time in the 3DS's lifecycle and that certainly helped with its success, but I feel like Fates did nothing to capitalize on that momentum. Its launch had people whining about "censorship", localization issues (whether warranted or not), complaints about selling the different routes separately, etc. It tried to expand on what made Awakening successful with mixed results like that weird Amie mini-game (because audiences loved supports so let's give them even more shipping opportunities, no matter how out of place the whole thing feels) and shoving in child units because that's what worked in Awakening. Gratuitous cameos, recycled designs for fanservice's sake, sometimes it felt like a game with an identity crisis. I love Conquest for its gameplay, but the whole package is kind of a bloated mess. It all seemed a little too much even for Awakening's audience, I remember how a lot of people were saying they had hundreds of hours on Awakening but were already burnt out from Fates at the end of their second route. Echoes coming right after with little to no fanfare also doesn't help, but hopefully that's just because people moved on from the 3DS in general.
As for Awakening, its success comes down to a likeable cast of characters, a character-focused narrative to complement said cast, lots of polish and QOL improvements over past games, presentation, casual mode, make your own avatar, and an increased focus on shipping. People like to joke about waifus but the latter was definitely a big factor on the game's success. Audiences like seeing characters they care about getting together, specially if it has both gameplay and story repercussions (you get rewarded for shipping your favorite characters by getting even more likeable characters, each with their own set of conversations and pairing options) and one of said characters also happens to be sort of a self-insert. I mean, even Bioware throws that stuff in their games because they know it works. Timing was important, yeah, but that alone doesn't explain how the game took off. Awakening was a more commercial product than past games and I doubt something like Radiant Dawn or New Mystery of the Emblem would've been as successful in its place.