I still don't understand why they couldn't just include Lords/royals from all mainline games. Even if it were just the main protagonists from those games, I don't think there'd have been as big a criticism/backlash. Makes it even more confusing given Nintendo were more than happy to fill up Heroes with characters from past releases.
Heroes and Warriors are two different games with different focuses. Heroes is a gacha game where part of the experience is the gacha collection element and the roster can be updated for as long as the game is live. Warriors is an action game where the roster has to be capped realistically at some point, and thus choices have to be made in terms of what games get represented and which characters from those games are included. FE Warriors has one of the largest casts of player characters of any Warriors game based on a third-party license at about 23 characters if I recall right (25, if you count the Robin and Corrin gender alts), so it's already being generous in that regard. This total also includes the two original characters and does not take into account any of the post-launch DLC.
If you take the royals/lords from each of the games, that's Marth, Alm, Celica, Sigurd, Seliph, Leif, Roy, Eliwood, Hector, Lyn, Eirika, Ephraim, Ike, Micaiah, Chrom, Lucina, Robin, and Corrin. That's eighteen characters, and of those, thirteen are sword users, two are lance users, one is an axe user, and two are mages. The roster of the actual game leans heavily on sword users, as well, but not by this ratio. Also, of the lords/royals listed here, the vast majority of them are foot units, and there are no flying units.
So how do you fill in the gaps, then? Let's say with 18 characters, there's room for five more for a total of twenty-three. So let's pick Caeda (flying lance) and Tiki (manakete), who are in the real roster. That's twenty. That leaves you with three more slots to divide between the entire series. Favorites are going to get left out whether you like it or not.
Focusing the roster primarily on three entries was the smart decision. It let them dig deep into each of the three games, as well as write a narrative that was more coherent for the fact that so many of the characters were already familiar with each other. (By contrast, Dynasty Warriors Gundam 3 has a mishmash roster and a story that is shoddy nonsense, at best, to the point that characters are only able to spout catchphrases in battle with minimal story value.)