@bowlofchilli3 has explained the constitutional and philosophical reasons.

Now let me give the same thing from a more practical or political point.

Within any party there is a broad range of views, and the views that dominate in any one party are constantly shifting back and forth. Sometimes there comes a time when a Joe Lieberman or an Arlen Specter without really changing their own views at all feels they can't stay in their previous party, because it's moved away from what they felt they signed up for.

As politicians like to say in those circumstances: "I didn't leave my party, they left me." Now if the voters did in fact select the individual for their views, and not just because of their party label, there is no reason why they should have to face re-election.

And the places that elect people centrists who might cross party lines like that generally know what they're getting when they vote, and would never have elected less moderate candidates of either party anyway.

They can switch because the United States uses a political system that is based on individual candidates rather than parties. Although candidates claim to represent, and are often supported by a specific party, the candidate is running on his or her individual merits and beliefs. Voters elect the person, not the party.

In contrast, for example, many European countries employ a system in which voters actually vote for a party, and not a specific candidate. The winning party organization is the sole decision maker with regard to who will be its representation in government. The voters don't choose the people, they choose the party.

That said, United States elected officials may switch parties whenever they choose. However, most choose not to so as to not upset voters whom they depend on for reelection.

Our country evolved at a time when there were no political parties in place, when the Constitution was ratified. As such, under the impetus of the grass roots from the historical basis for forming the constitution (England's rule upon the States), safeguards were enacted that exist today, even with the dynamic changes that have occurred since the framers contemplated and created the Constitution.

There was also an underline reason that senators were NOT directly elected by the people - clearly outlined in the Constitution - to be selected by the states' legislatures.

Thanks to the 17th amendment, that reason (and states' rights, for all intents and purposes, regardless of the intent of the 10th amendment) was lost.

While, ethically, switching parties is void of integrity, especially when campaign money was induced on the promise of representing "party interests, " such switching can not be the basis for legal recourse, with respect to the available remedies (i.e., impeachment or sanctions). One caveat, if the fine people of the great State of Pennsylvania were to gather enough signatures and obtain a referendum, then achieve enough votes, it is my belief (from vague memories of Constitutional Law Claw, then they could remove the Senator from office.

Enclosed is a source article for this question, regarding the United States system of representation, party politics, explained in an evolutionary chronology of understanding where we are now and how we are where we are presently, with respect to our political system and the Constitution.

http://www.answers.com/topic/direct-democracy