There's a couple of reasons:
1. Intrusiveness. The War on Terrorism hardly affects most people in their day-to-day lives - they have to take off their shoes at airports, nameless bureaucracies have computers read thru their humdrum emails and it was a very defined subset of Americans who were shipped off to war. The War on Carbon potentially affects everything because it makes day to day life so much more expensive and restricts normal consumer choice (you want a filament lightbulb? Too bad - buy a CFL! Don't like them for some reason? Buy an LED for 10x as much! Oh, and did we tell you that your yearly trip to Grandma in Arizona is killing the planet? Stop doing that!).
2. The sheer cost. Most mitigation schemes for global warming are in the ludicrous number range - trillions of dollars a year for 100 or more years. On top of that drag, most are designed to destroy economic growth (you almost have to in order to ratchet down energy use fast enough). This isn't a boo-hoo First World Problem - it's mostly a tragic Third World Problem. Germany gives up a few percentages of economic growth for 50 years - that's a hit (and the government would fall). Ethiopia gives up economic growth for 50 years and you're consigning millions to abject poverty and breeds radicalism.
3. The perception gap. The War on Terror seems to affect the rich and powerful in the same ways it affects the poor and the hoi-polloi - maybe a little less (they're rich!) but it seems somewhat similar. Even Mitt Romney has to take his shoes off at the airport. AGW mitigation, however, seems to be a problem that only the poor and middle class need to sacrifice for - Al Gore has numerous mansions, jets all over the world, uses more energy in a day than most families do in a week. AGW "solutions" seem to nicely dovetail with the natural desires of the elite - less upward mobility, pricier and/or more organic food, paternalism toward their lessers.
The optics on AGW are terrible - which is one reason there's such resistance. Killing bad guys, however expensive and destructive that may be, appeals to a lot of folks. If there were better optics - and a range of policy choices that didn't seem to favor the technocratic elite - you might not have such hostility.