In part this is because unlike now we didn’t have a media complicit in American defeat in WWII. There’s no question that the media have a significant effect on the rules of engagement.
On the contrary. The media was ambivalent about the Iraq invasion, and somewhat supportive of our involvement in Afghanistan. Embedded reporters were popular and had glowing reports to deliver while we were engaged with Saddam's "forces."
It wasn't until after the President failed to explain the situation with WMD adequately, and then engaged in the present policy of confining our engagement to Iraq and Afghanistan while the Saudis, Pakistanis, Iranians, and other Islamic countries support our enemies around the world -- that the press really got its anti-American legs.
If President Roosevelt had waited much longer to open a land front in Europe, or if we had failed to utilize stragegic bombing during WWII, the press might have taken a different tact. Americans like winners, and the situation in Iraq is less than "victorious." The casualty rates are tremendous, and it's basically for an ill-defined objective that few Americans can still understand.
This is why the Democrats and the press corps are finding so many ways to undermine the President.
FWIW, I don’t think America will ever win another war until we lose our squeamishness about killing people.