Ultimately, the Sunnis realise the only way to be rid of the US* is to go for a political process, because the US wouldn't be able to refuse a request to leave by the Iraqi government. I think it's a question of status quo, or anything else, and it makes sense for them choosing the 'anything else' option. Because at the end of the day, even if the constitution was scrapped, it'd still be a majority Shia/Kurd coalition dictating the contents of the next version.
*of course, the US isn't the only country there, but it's the one engaged in the most combat and definately the most within Sunni majority areas.