Why does Richard decline taking the D'haran Throne if he knows having an army behind him would be really helpful?

He doesn't want to go by the name of Rahl or have anything to do with him.

yes and also he doesnt want any power. he doesnt think a life behind a throne is for him.

I think that he dose not want to be ruined by the power that comes with the job and lose who he realy is, also he could be worrying if he would turn out like his brother. In my mind he dose not think that he would be the right person as he would have to many other responsability`s that will get in the way of helping others and doing the right thing to save everyone from what he helped cause, he feels responsible for what has hapened after he defeted Darken Rahl. He feels he is doing the right thing.