The thing is that Nintendo doesn't cares.
Yes, FE isn't a titan compared to Mario, Zelda, Animal Crossing and Pokémon, and even the two Splatoon games beated the series in sales.
But it has also been a series that in the last decade, has brought them much more income than many of their more historically renowned series; Three Houses by itself sold more than Kirby Star Allies, which is the second best selling Kirby game, and it also has sold more than the port of Tropical Freeze or Yoshi's Crafted World. The lowest selling game in recent years has been Shadows of Valentia, which even then outsold Samus Returns.
And then there's the gacha juggernaut called Fire Emblem Heroes, which is at least amongst the Top 20 most grossing mobile games of the years it's been on, which sometimes placing close to money-making juggernauts such as Clash of Titans, Pokémon GO, Fate/Grand Order, Candy Crush and Mosnter Strike.
tl;dr, to Nintendo it has become a pillar to the company and of course they're going to give it priority over others.