It is much harder to do. We all speak the same 24 hour clock and it is not a culturally significant thing. It would be nice if everyone could understand everyone but there is something to be lost by losing languages. There is absolutely nothing to be lost by doing away with timezones.
Of course there's something lost. If I see my colleague's local time will be 9:00PM, I know not to schedule a meeting then. It's a method for translating familiar associations of hours across space.
Also, and more importantly, "it's 5:00 somewhere!" would cease to be true.
