Andrew Gelman tries to answer:

Being a moderate is worth about 2% of the vote in a congressional election: it ain’t nuthin, but it certainly is not a paramount concern for most representatives....Incumbent congressmembers almost always win reelection.  And, when they don’t, they’re often losing as part of a national swing (as in the 1994 Republican sweep or the 2006/2008 Democratic shift).  And when an incumbent does lose unexpectedly, it can be for something unrelated to their votes (remember the “check kiting scandal” of 1992?).

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