Not sure I follow your version of history. Powell presented the Iraq WMD case to the U.N., and very effectively for all of us listening (except the U.N.).
In recent times Powell has said he regretted his presentation to the UN.
Powell was always the consummate politician--as somebody who navigated his way through the Pentagon's Byzantine corridors all the way to JCF Chairman would need to be. Consequently there's a lot of history about him not known except to those who follow the news closely. In the first Bush administration Powell repeatedly positioned himself to appear publicly as a moderating but loyal voice in the White House. In that role, the famous UN presentation was a task he couldn't decline. But the truth was he often used his network to leak damaging information and manipulate foreign policy in ways not very satisfactory to the President nor to Rummy and Rice.
Anybody who doubts this should read this (accurate, but not verbatim) exchange between the lines: "Mr. President, I know with all the changes in the second administration things will be hectic, and I'd be willing to stay on as Secretary during the transition." Bush: "Mr. Powell, you've been a fine public servant, but that won't be necessary."