Dropped from the line-up in 2001 the Cadillac Catera was an "Americanized" and re-badged Opel Omega. Unfortunately Cadillac's version of Americanizing was making it soft and squishy with low power output. A pity, In Opel trim they're pretty nice.
The reasons we don't see any Opels today are the same reasons they went bye-bye in '75, strong Mark, er... Euro, weak dollar and little interest from GM marketeers.
Small German cars are too tough to bring over here and sell competitively. German marques like BMW, Audi and Mercedes only sell medium and higher market models. Even Volkswagen is pushing their Touregs and Phaetons because they can't make enough money with Golfs and Beetles.
Opel doesn't make an SUV (they sell Isuzu Rodeos in Germany badged as the Opel Frontera) so GM doesn't have high mark-up vehicle to bring in. If they tried to bring over the mid and higher Opels they'd be fighting in an already crowded premium car market. Heck, they just killed Oldsmobile because they too many midsize and large cars in their stable.
Strangely enough the only car you can buy in the US today that approximates what you can get of the showroom floor in an Opel dealer is the Lotus Elise. Sold as the Opel Speedster and Vauxhall VX220 Lotus OEM builds them with different bodywork and GM powertrains. Of course they aren't German cars at all, they're LBCs!
PC.