1ºLiz good question, and I can't/won't claim I know how they're making decisions, but I've observed a few trends. The big one is that they're not going to make something if they can't sell it, even if they experiment with it for a few years before quitting.
Emissions regulations used to largely ignore the diesel market. Growing up in California in the 80's, we had "smog testing" before most of the rest of the country. This was back when you could buy a "49 state" model, or a "California" car with more emissions equipment. I had a friend who really excited to get his hands on a diesel powered Isuzu mini-truck. It was exempt from emissions inspections, and his plan was to swap in a gasoline V8, because "no one at the DMV will ever know."
Eventually after years of gasoline emissions restrictions at the federal level, it made environmental sense to tackle diesels. Passenger light diesel applications were kind of a testing ground before expanding into larger platforms like 1 ton trucks and large commercial vehicles.
It's not totally surprising to see a European brand like VW was the first to get in trouble for "cheating" on US emissions requirements. The Europeans have had a history of subsidizing diesel fuel costs, with lower taxes being one of the mechanisms in place. They've got more experience with these engines and have done a lot to try to popularize "passenger" diesels in the US.
The big three American manufacturers still seem to be finding their footing. Chrysler seems to be having the most success with their 3.0 liter V6 Ecodiesel platform showing up in 1/2 ton trucks, but also in platforms like the Jeep Grand Cherokee and the Wrangler. Ford has a 3.0 diesel in the F-150 but it doesn't seem to be selling very well. GM was last to the 1/2 ton diesel truck market with their new 3.0 inline 6 cylinder in the 1500 series trucks along with the Tahoe & Yukon.
GM, however, ran even smaller diesel engines for a number of years in smaller trucks like the Colorado & Canyon, along with the Equinox SUV. Even he Chevy Cruze had a little diesel engine as an option for a few years. It looks like they didn't have enough sales volume to justify keeping the engines around.
Maybe it's just poor sales driven by the American perception that diesel is for trucks and tractors? It would be an interesting study to see.