Why Does Yom Kippur Come after Rosh Hashana?

Yom Kippur is only the end of teshuva (repentence). Rosh Hashana is the chief teshuva for forgetting about Hashem. After we finish the great teshuva in believing in a Creator, a King who controls the universe and everything is under His supreme wisdom, then we're ready to do teshuva on the details, this sin and that sin.  That's the business of the Days of Awe and of Yom Kippur. -- Easy Road to Teshuva (#524)