There are plenty of examples from history and now of better governed countries. I don't know how anyone can look at the US and think it's success is because of constitution and not from being the 3rd largest country on earth with a land empire full of abundant resources that it's never given up and successfully assimilated via imported populations.
why would you muck with one of the most complicated systems humans have ever created on the off chance you fuck everything up when the current system has made you the most successful civilization in human history and has done so for 250 years.
i mean is it really hard to imagine why Americans might be wary to change things? maintaining a stable civilization is a pretty precarious undertaking.