AN ENDING propaganda: It builds on the emotional impact of the original RUINS theme to deliver a sombre tone and relay the events caused by you, but also incites a glimmer of nostalgia for the earlier stages of the game, as well as providing an emotional backdrop for either talking to your beloved friends one last time or feeling empty due to the sins you committed in this life.
Waterfall, while being an atmospheric zone theme and providing original feelings and sounds, offering courageous suspense for your inevitable fight with Undyne, doesn't carry that same retroflection that is core to the Undertale experience.
also i personally like an ending a lot more