The reasons why Americans are politically gullible are many....
First, it all starts from childhood by getting bombarded with slogans and indoctrination of this mythical place known as America "the best country on earth". Well if we're already the best country on earth that means everything is the way it should be. There's no need to improve anything, we're already there.
Then you're raised in a public school setting that reinforces that message and shys away from teaching the real blood and guts history of the evil doings alongside of the good that this country has done. After all, no one wants to feel bad about the place they call home.
We also are enmeshed inside a consumer/celebrity/sports/religious culture that diverts needed attention away from social/economic issues that effect the lives of working people along with the 24/7 propaganda message from the owner class of society that convinces normal everyday working Americans to vote against their own best interests.
When it comes to politics (i.e. issues that have real consequences upon their lives), Americans are by and large, bored. They'd rather go shopping.
This is why you see wild swings in polls. Americans don't pay attention. They can be swayed. Europeans on the other hand are more informed politically. You don't see wild swings in polls in Europe because for the most part they are more informed and know the issues so propaganda tends to be not as effective there as it is here.
Europeans also tend to know when their ox is being gored and protest in the streets more than Americans do.
