Because no occupying power has ever won Afghanistan. The British tried, the Russians tried.
The greatest massacre of British soldiers in the history of the British empire happened in Afghanistan.
"Slaughter in the Mountain Passes
A magazine based in Boston, the North American Review, published a remarkably extensive and timely account titled “The English in Afghanistan” six months later, in July 1842. It contained this vivid description (some antiquated spellings have been left intact):
On the 6th of January, 1842, the Caboul forces commenced their retreat through the dismal pass, destined to be their grave. On the third day they were attacked by the mountaineers from all points, and a fearful slaughter ensued…
The troops kept on, and awful scenes ensued. Without food, mangled and cut to pieces, each one caring only for himself, all subordination had fled; and the soldiers of the forty-fourth English regiment are reported to have knocked down their officers with the butts of their muskets.
On the 13th of January, just seven days after the retreat commenced, one man, bloody and torn, mounted on a miserable pony, and pursued by horsemen, was seen riding furiously across the plains to Jellalabad. That was Dr. Brydon, the sole person to tell the tale of the passage of Khourd Caboul.
More than 16,000 people had set out on the retreat from Kabul, and in the end only one man, Dr. William Brydon, a British Army surgeon, had made it alive to Jalalabad. The garrison there lit signal fires and sounded bugles to guide other British survivors to safety, but after several days they realized that Brydon would be the only one. It was believed the Afghans let him live so he could tell the grisly story. "
Obama's making a strategical mistake by sending 30,000 troops to Afghanistan.