You may be interested in past posts on this topic:
After the Empire of Japan's defeat in WW2, why wasn't Emperor Hirohito put on trial for war crimes?
^(26 Feb 2014 | 3 comments)
^In ^(this answer)^(, /u/phoenixbasileus gives a detailed overview of Allied motivations and planning surrounding the matter.)
^In ^(this answer)^(, /u/tayaravaknin gives a lower level analysis of American policy.)
Why wasn't Emperor Hirohito tried for war crimes following WWII, and what reasons did General MacArthur have for steering things this way?
^(06 Jan 2014 | 4 comments)
^(/u/TheWinStore details how the matter fit into US post-war goals using primary source evidence.)
For followup questions, use username mentions in this thread to direct questions to specific commenters.
Short answer: Because McArthur personally and through him the American occupiers in general believed in keeping the Emperor clear of blame and using the Emperor as a source of stability and legitimisation of the new constitutional democratic order.
Contrary to what some of the unsourced replies here are saying, this was not an objective opinion nor something broadly shared by everyone involved. There were and still are those who seriously question this move and who believe at the very least the Emperor ought to have been made to abdicate as a way of accepting culpability for not just the war with the US but the entire era of militarism leading up to it. Examples are historians John Dower and Herbert Bix.
For further reading I recommend the comments linked by /u/OldWorldGlory, and Embracing Defeat by John Dower.
It came down to two questions at the end of the war, 1) what was his actual role in conducting the war. Was he a figurehead, an icon with the general staff actually calling the shots or was he actually involved in the strategy of the war. 2) Post war the US needed him, they needed him to validate the US occupation and to provide continuity of leadership with the general population. Most modern historians feel that he was wholly removed from the tactical level in the war and even at the strategic level had very little input, indeed when he announced the surrender of Japan there were segments of the Army which fought back and tried to block his announcement that Japan would surrender. Post war MacAurther knew he'd need him in the rebuilding process and to get the Japanese government and society working again, Japan was utterly decimated at the end of the war.
TL;DR - it's not clear he really had much to do with the war and the US needed him after the war.