Why do we have different time zones around the world?
our planet rotates, so dawn and dusk occurs at different times around the world. we cant have someone waking up at at midnight cuz their clocks says it's 8am or have someone going to sleep at noon cuz their clock says it's 10pm.
Would be pretty weird if the sun rises at midnight in some places, wouldn't it? In fact there is a unified "planet time", but people can't get used to use it.
Because the earth is rotating. Japan faces the sun first. It takes ~20 hours for US to reach there.