The Wii in general was not a good console for Fire Emblem; despite its large install base, the majority of Wii owners were casuals that just bought it for Wii Sports, Wii Fit, Mario Kart, and NSMB, with them only maybe buying a couple of the other big Nintendo releases or the myriad of casual slop on the console, leading to low attach rates for Wii games outside of the big casual titles. A big budget Fire Emblem game was never going to succeed on the Wii no matter how good it was, especially when it wasn't adjusted in any way to try appealing to the Wii's casual-oriented install base like some other more hardcore titles were (e.g. Twilight Princess incorporating the Wiimote into its control scheme and Skyward Sword being fully motion controlled, Mario Galaxy and Donkey Kong Returns incorporating pointless motion control, Super Smash Bros Brawl's infamous casualizing, etc.). There was that infamous review of Radiant Dawn from that time which criticized it for having no motion control nor mii support, which is funny to look back on now, but it is the sort of attitude your average Wii owner would have had at the time.
The DS had a similar problem of its massive on-paper install base being largely casuals, so it wasn't an ideal platform either for Fire Emblem games, though being cheaper to produce them for DS meant Shadow Dragon and New Mystery weren't the big financial failure that Radiant Dawn was.