>>721095107
Speaking as a casual, it likely had to do with the lack of paired endings. Everyone likes a happy ending and shipping their favourite characters.
Putting Fire Emblem front and centre on the 3DS gave massive audience appeal for that, then it was taken away in Echoes (remake of Gaiden, which didn't include endings for supports, let alone supports themselves).
These expectations were met again in Three Houses, which followed the sales trend set by Awakening and Fates. Yet this feature was taken away yet again in Engage, despite not being a remake. Many new fans introduced by Three Houses on the mainstream Switch console still bought it, but the much larger "story game" audience saw no need to buy it.
>tl;dr
Fire Emblem is now a romance novel series. Attempts to return to "romanticism" will isolate the new, larger audience.