Some of the most popular units from Fire Emblem: Three Houses have joined the ranks of the Fire Emblem Heroes. It was only a matter of time before this happened, of course, but the characters chosen give us a certain insight into the developers' mindsets. After all, Intelligent Systems develops both games. This means they have, for the most part, total control over which characters make the jump to Heroes. The fact that they chose Hilda (and Mercedes, Petra, and Hubert), shows they expected the character to be popular with fans.
Fire Emblem Heroes is a mobile game that acts as a sort of "all-stars" gathering of Fire Emblem characters. As is typical of these types of crossover games, the most popular characters tend to be added first. That's why the game started with a banner promoting the likes of Robin and Marth, both Smash Bros. That's why Camilla, Fates' waifu supreme, has more alternate costume versions than several other characters put together. When it came to Three Houses, the main characters were already added when the game launched.
A new trailer just dropped for the second batch of Three Houses units. With the main characters out of the way, three of the most popular ladies are given a chance to shine.
It's worth noting that Intelligent Systems seems to design characters for Heroes well ahead of time. Fans have found clues for seasonal units in weeks before they launch. So it's very likely that Hilda and friends were chosen long before Three Houses released. Intelligent Systems would have had to guess which units fans would want first. They had some clues, with fans eagerly latching on to every tease and leak about Three Houses and declaring waifus and husbandos in preparation for launch. Still, these picks are mostly on point for being advance guesses.
Hilda of course has been the most popular Golden Deer girl, thanks to her ridiculous axe skills and charming laziness. Mercedes is by far the Blue Lions standout, and a priority recruit for most. Petra, meanwhile, has some fierce competition in the Red Eagles. Bernadetta and Dorothea seem just as popular, maybe even moreso if the deployment/recruitment statistics the game shows you are accurate. Still, Petra is popular enough to pose a threat to your orbs stash.
Hubert is the one outlier here. While he certainly has his fans, there are other candidates for token dude. Linhardt would have probably been a better choice, what with him being the son of Lyn and Reinhardt (or so the memes tell me). But hey, at least the Death Knight is coming in a Grand Hero Battle.