When you ask questions like this, it is much better to put them in the Discussions section so that you can get a wide variety of answers.
Many people have no confidence in Congress because they believe that members of Congress do not really care about the welfare of the United States as a whole. Instead, they believe that members of Congress are out to promote their own careers or, at best, to get their party to win at any cost. People see that members of Congress change their minds on issues (Medicare is a big one for this) depending on what would benefit their party. Democrats were for cuts to Medicare a couple years ago and Republicans were opposed. Now that dynamic has switched. When Americans see things like this happen, they become cynical and this helps them to lose faith in Congress.