Probably two or three reasons. First, the diesel variants are considerably more expensive than the gasoline counterparts, which, at the lower price points of the Jetta and Golf, really tends to stick out. Second, most older Americans remember the 70s/80s diesel failures and that stigma sticks. The younger ones, those with skulls of mush, anyway, think diesel is on par with burning piles of used tires.
Lastly, in some areas, diesel prices, combined with the higher purchase price of the car, don't turn people on to the fact they get far better mileage than their gasoline counterparts, and they can't do the math.
I'd take one if I could afford to, but right now, I'm saving up for a diesel pickup and a travel trailer, my car is paid for, and my commute only costs me a tank of gas a month, so there's no incentive for me to buy a new diesel.
I agree with your analysis.
I snooped around at a few VW sites and apparently, they're not offering the TDIs right now. They probably will after this blows over a it. You could maybe buy a used one real cheap but I would only want one that had NOT been modified to comply with the do-gooder laws. The modified ones are probably absolute dogs as far as driving goes.