Nancy Pelosi represents the 8th Congressional District, which is basically San Francisco. She knows who her constituents are.
I suspect some other reason for her stance on this (can't fathom what, though).
It's unclear how motivated her constituents are against it, but it seems likely that all that letter writing may be paying off. In extreme cases, sitting members can be vulnerable to losing their nomination within the party (eg http://en.wikipedia.org/wiki/Joe_Lieberman#2006_Senate_elect...).
Edit: Downvotes? I'm not from the US - what did I get wrong?
Making matters worse, voters tend to want conflicting things. For example, in general they want more government programs and also lower taxes.
Making matters even more difficult, what voters say they want and what they actually want can be different stories. Schwarzenegger got elected on the premise that he was going to do certain things. He tried to do those things. He was shut down by everyone- including directly by the voters.
Do you have a citation on this? I recently read Freakonomics where the authors claimed to be able to prove this theory false, but I didn't look up their numbers.
But that only work when their constituents are insuffiently motivated to actively target them to remove them from power.
Members are quite vulnerable to losing their party nomination as it takes less votes than at an election. That was how the tea party got members elected (although usually that wasn't against sitting members)
That isn't praise, I personally don't like the guy's politics much. But clearly he pays attention to the needs of the large employers in his district. I think it's on target to point out that many of the large employers in her district oppose SOPA. That probably does have some influence on her stance.
It is true laws that are good for your employer can be good for you, because you of course have a vested interest in your employer. But I think many people feel it should be the other way- i.e. representatives shouldn't be looking out for business and hoping to cover the voters, they should be looking out for the voters, who will do the worrying about their employers. The way you illustrate, the power dynamic is in favor of business. Some crazy people feel it should be in favor of voters.
How to explain the widespread practice of earmarking, then?
The first two deliver the third.
Pork (bits added to bill that affect something in their electorate) affects the second and third, and was probably delivered as the result of a contribution from a lobbyist who was retained by a corporate.
I cannot recommend Larry Lessig's latest book enough to US voters.
So, like she says, "we need to find another way". They'll come up with a new bill that is either less objectionable or one that hides the objectionable parts more clearly... and then they'll pass it when nobody's paying attention.