Not too long ago there was a picture getting passed around of a city building undergoing exterior renovations. When construction pulled the 1950ish exterior off they found forgotten, beautiful mouldings from 1900ish underneath. New generations are always going to want to replace/cover up the decorations/style of the previous generations.

I imagine mosaic floors were common enough at one point people didn't particularly think to try to preserve them. They fell out of fashion and people intentionally covered them up like painting over ugly wallpaper in an old home. Maybe an elderly, disabled, or particularly lonely person, last of their family, last owner of the building, wasn't able to really keep the floor clean. Once enough dirt collected, no one would be the wiser after the last person to know died.