Good riddance! The Hummer is the epitome of the impractical car. It's heavy, it's huge, and it's impossible to park. The idea that any of these Hummers would actually be used off-road anywhere was ridiculous in the first place. (Perhaps there's one out there somewhere?)
I think what partially contributed to its decline was the environmental stigma. The Hummer is easy to spot and symbolizes wastefulness and excess to many environmentalists. What I find interesting, however, is that there are still plenty of cars with worse mpg that are also never taken off-road. So why is the Hummer the only one that has left production?
The H3 averaged around 16 mpg. Sure, it's bad, but it's comparable or better to the city milage for the Nissan Titan 4WD pickup (12 mpg), the Mercedez Benz ML 63 AMG SUV (11 mpg), or the Chevrolet Express 1500 (13 mpg). According to fueleconomy.gov, these are actually the least fuel-efficient cars on the road. Unlike the Hummer, they're not stigmatized. They have perfected the art of blending in with some of the most fuel efficient cars, like the Ford Escape Hybrid SUV or the Mazda Tribute Hybrid SUV, both at 34 mpg. http://www.fueleconomy.gov/feg/bestworstEPAtrucks.htm
Of course, the recession probably hurt the Hummer the most. It's one of the most expensive road-vehicles, and with gas prices so volatile, it's one of the worst investments one could possibly make.