Psychology. Why it is we hold to beliefs and habits without evidence is a hard question that probably takes several books worth of material to answer :-/
But it is true that change is most often very slow. It isn't only management. It is our psychology that then manifests itself everywhere, even the least likely of all places - science.
A new scientific truth does not triumph by convincing its opponents and making them see the light, but rather because its opponents eventually die, and a new generation grows up that is familiar with it.Max Planck
Ironically in management we often also seek the new new thing. So while great ideas take a long time to become common practice we stop looking at them fairly quickly because we decide they are old outdated ideas. Not a very effective strategy :-(
Related: Change is Necessary, But it has to be the Right Change - Change is Often Not Improvement - When the ideas have reached the level of habits you have changed.