Because everybody is already used to telling time in a certain way, and doesn't want to take the time to learn a new one, put in the effort to learn a new one,

Because everybody is already used to telling time in a certain way, and doesn't want to take the time to learn a new one, put in the effort to learn a new one, or deal with the initial wave of missed meetings and general confusion that would result as larger portions of the population moved to a different way of telling time.