There are many more factors that go into a game's sales than just marketing.
The biggest thing that held Fire Emblem back, besides marking, was its permadeath feature in its games. Keep in mind, the series up to that point was a hard series meant for more experienced players. Hell, games like Radiant Dawn were way too damn hard for their own good. So to add a permadeath feature on top of that, that scared off potential newbies to the franchise until Awakening made it optional and toned down the difficulty.
As for Metroid:
Consider that Prime 1 was on a limited install base in the GameCube, which as a borderline stinker in sales. And Prime 3 just wasn't the kind of game that appealed to casual players, the vast majority of the Wii's install base.
Dread doesn't have either of those problems. The Switch is more successful than either the Wii or GameCube were and will go on to outsell both of them combined (and by a comfortable margin from the looks of it). And it's a far more popular system amongst dedicated, hardcore gamers than the Wii ever was.
Permadeath was made optional in New Mystery of the Emblem, the DS remake of Fire Emblem 3 that preceded Awakening. Radiant Dawn allows hard saves anytime during the player's turn, at least on easy and normal difficulty; it's only on hard mode that there's only the typical soft save available. Shadow Dragon, the DS remake of Fire Emblem 1, provided two hard save points per chapter from chapter 2 onwards. So no, I don't believe it that permadeath was that much of an obstacle before Awakening. Path of Radiance added a West-exclusive easy mode which is Japan's easy mode with 50% higher EXP gains; characters level up so fast that much of the challenge becomes trivial, so even newcomers don't have to feel anxious all the time.
Before Awakening, even most Nintendo fans only knew of Fire Emblem through Roy and Marth in the Super Smash Bros. series. Only the name though, not what the games were actually about. That's why marketing made such a big difference for Awakening.
Metroid's sales were never big enough to be hindered by an installed base as low as the GC's. Metroid is a series that appeals to the experienced gamer, so a console that sells first and foremost to experienced gamers has the majority of potential customers on board. It was also Metroid's first 3D outing, so a sales boost over its direct sequels (Echoes and Corruption sold about the same) doesn't require a bigger look at the consoles the games were released on. Likewise, Switch isn't going to provide much of a boost to such a game; we already know that Fire Emblem didn't do much better than on the 3DS. The increase in what you call "dedicated hardcore gamers" will come down to Switch being released 11 years after the Wii, so the total number of experienced gamers has naturally grown over time.
Hollow Knight's sales don't mean much. It was a $15 game that has also gone on sale plenty of times. At these price levels people hardly think about taking a chance.
What you have to consider is that Nintendo didn't mention Metroid Dread during their financial briefing. They usually mention recent releases that missed the shipment data cut-off when they've done well and are contributing to the business in a noteworthy way. That's why it's fair to assume that Dread has done just okay and isn't on track to set new franchise records for sure. Perhaps time will be in Dread's favor, but that's up in the air.