Because we live on an approximately spherical world whose rotation is not tidal locked to the star it orbits, that world rotates around an axis that is not perpendicular to its orbital plane around said star, we have a biology that uses sunrise to synchronize its internal clocks to the planet's rotation, we have spread to occupy regions of the world that are not near its equator, and we align our timekeeping to midnight or noon.
That might be a valid reason, but it's almost certainly not an explanation of why things are the way they are now or why they might change in the future. I don't recall there ever being a vote on the subject.