Could it be as simple that 64 keys and speaker versions are aimed for the home
market, and 76 keys and no speakers are
aimed for the "pro" market, who mostly
have PA systems to carry the sound?
Also I guess that 76 keys are preferred when play "pianomode"...
Cheers 🥂
"Success is not counted by how high you have climbed
but by how many you brought with you." (Wil Rose)