The average for Prii listed here is at or just below the EPA combined rating. 50mpg for the gen3, 46mpg for the gen2, and 41mpg for the gen1. The gen1s are averaging above the combined, but those were mostly bought by the fuel conservers at the time.
I didn't read to see if anyone said this, but in reagards to the original question, I think a lot of it has to do with A) The Jetta (and other VWs) have the American diesel car market cornered in recent years, and B) The sort of people who buy German diesel cars in N America are typically eccentric math/engineering types to which a site like this caters perfectly.
TDi Jettas were the domain of your stereotypical professor where I was in California. Here in Texas though, there is a familiarity with diesels, thanks to the ever-popular diesel pickup. I see plenty of "regular" folk in TDi VWs here, young, old, unabashedly average people with none of the professor air being thrown off.
TDis are incredible fuel-misers though, and I don't doubt that most owners see the (relatively) eye-popping economy of these cars and get a little fire lit under them.
The problems is that in the US, you guys tend to get the "normal" versions of the TDI's, VW is too afraid to sell the eco versions (Blue motion as its known here) as the performance is considered too slow. As fuel is so cheap there, people aren't as willing to compromise performance over economy, even though the eco versions get 15 - 20% better economy.