I don't think it is coincidental that Hybrid sales peaked in 2009 and the tax incentives ended between 2007 and 2010.
http://www.hybridcars.com/federal-incentives.html "Toyota and Lexus hybrids became ineligible in September 2007. Honda lost its eligibility on Jan. 1, 2009, and Ford's expired March 31, 2010."
Informed buyers compare the total cost of ownership at one of the many sites available online. For example, comparing the Ford Fusion sedan to the Ford Fusion Hybrid sedan at
http://www.edmunds.com/tco.html the cost to own and operate the Fusion sedan for 5 years is $41,624 compared to the Hybrid at $47,465 or $5,841 more to drive the Hybrid based on a 5-year estimate with 15,000 miles driven per year. Although the hybrid saves $4,075 in fuel, it costs $12,607 more to buy so depreciation, taxes and fees and financing costs are higher, but fuel and insurance are lower. BTW, the Ford Fusion Hybrid breakeven point is around 150,000 miles. When the tax incentive was still available the lower cost Hybrids dominated sales because their cost premium was less than or close to the tax incentive. Now that the tax incentive is gone hybrids don't make economical sense.
I bought a Honda Civic GX compressed natural gas car in 2006. The CNG premium was $4,000 which matched the tax incentive so my after taxes premium was zero. Since it is an ultra low emission vehicle, I got the silver HOV lane stickers which are good through 2015. I get 34 miles per equivalent gallon, but the cost of fuel is $2.15 per equivalent gallon. So the fuel cost for my 64 mile daily commute is $4.30 in the Civic GX versus $18.50 in the H3T. So I save $14.20 per day or $3,550 per year just on fuel cost. In addition I'm putting 16,000 fewer miles on the H3T per year and get to take the HOV lane for the next 4 years which saves me about 10 minutes per day (when I lived in Diamond Bar the HOV lane saved me 35 minutes per day).
If you look at the bottom on the
http://www.hybridcars.com/federal-incentives.html link there is a list of vehicles that qualify for incentive and there are 9 "clean diesel" vehicle incentives.