Because true forgiveness is all about repenting and turning away from your sins. Obedience is the act of turning away from them, because GOD has convicted you of your mistakes, and calls you to live a godly life.
Because you're missing the whole point if you just keep asking for forgiveness without changing your ways.
Because its by GOD's grace that we become obedient. We can go our whole lifes working without a lick of faith, and we wont have any kind of relationship with GOD. On the other hand, if we have true faith, we don't have to maintain a steady stream of works to 'be in' with GOD. It would please HIM if we served HIM whole heartedly, but its not "required". All we must do is believe. The act of believing is obedience.