Because if you show little opinion to the public until the public shows a strong opinion, it is easier to change your opinion. Most politicians gain favor by saying not what they believe in, but by saying what the public wants to hear (Clinton and McCain wanted to get rid of gas tax when gas prices soared). When one does this, you can't really stand up for stuff, and so you slowly become more and more spineless.

Because they're involved in capitalist politics that requires negotiation, secrets, power, etc.

They are not spineless and in fact they fight very hard for things that serve their interests. They are self-serving, myopic, and detached.
Most politicians are playing the game and getting along. There is an old Japanese saying "The nail that stands out gets hammered in." This is exactly what happens to people who try to stand out in politics especially at the national level. If you don't go along with what is being presented by the majority of your peers you will find that your projects will never get farther then your desk. The projects you can get people to sign on to will be killed on the floor and you will be receiving massive pressure from special interests to vote in their favor. The system is corrupted and anyone who tells you otherwise is selling you something. The goal of every politician is to get re-elected and they are willing to do just about anything to ensure they are. Unfortunately the people of the United States are left hanging out in the wind as a result. They don't see that the people are the reason they are in a position of power(At least in theory.) and that they are obligated to perform the duties of their office on behalf of those people and not the lobbies. They have allowed their perception of reality to become distorted and in their own minds removed themselves from the concerns of everyday Americans. They are concerned with the common American only as far as it relates to helping them maintain their position of privilege. They sit safe behind the palisades of power while people are getting evicted, starving, and being poisoned by the plutocracy they are currently representing. And, on behalf of my family I will say I am sorry. I'm sorry we did not fight harder to hold on to the republic. A woman once asked Benjamin Franklin " Sir what have you given us?" He replied "A republic mam if you can keep it." The reality is we have lost the republic. We are now in a democracy transitioning to oligarchy via a period of anarchy. May God have mercy on our souls.

They're voted into office, and they want to keep their listener's vote...whoever that listener happens to be.

There is also a lot of pressure from party politics. Toe the line, or you're out. Trying to balance that with the demands of constituents who don't necessarily agree with the party's demands makes many politicians resemble pendulums.