It's not so much that it happened, but how fast it happened.
It was always going to happen.
www.cnbc.com/2021/08/16/how-afghanistan-fell-to-the-taliban-so-quickly.html
"What’s key to note is that the Taliban did not have to fight their way into Afghanistan’s provincial capitals but rather brokered a series of surrenders, says Jack Watling, a research fellow for land warfare and military sciences at the Royal United Services Institute in London. Over the last few years of fighting, the group managed to gain control of some 50% of the country by seizing rural areas."
“The Taliban would infiltrate urban areas, assassinating key people like pilots, threatening the families of commanders, saying if you capitulate, you’ll save your family,” Watling said.
“A lot of people, because they lacked confidence that Kabul would be able to save them, capitulated.” More and more people chose this route, “so there was very little fighting, which is why it suddenly happened so fast,” he added.
“The speed is not a reflection of military capability, it is a reflection of a collapse in will to fight.”
"Kirsten Fontenrose, director of the Scowcroft Middle East Security Initiative at the Atlantic Council, said the Taliban has become more effective since the 1990s."
"Had the Taliban engaged in a full military onslaught and faced resistance, the blitz of the country would have taken longer — but it still would have happened, Watling believes."
“There’s never been a central government in Afghanistan. To think we could establish one was a fool’s errand,” said the former U.S. intelligence officer and Afghan War veteran. “The ‘surprise’ at the Taliban regaining power shows just how little Americans, from top to bottom, understand Afghanistan.”
"Tribal alliances in Afghanistan very often supersede national ones, or loyalties follow money and power. And part of the Taliban’s strength lay in the fact that as Pashtuns, they belonged to the largest ethnic group in Afghanistan."
"The nature of the U.S.-brokered cease-fire with the Taliban in early 2020 also further weakened the Afghan government’s image: Negotiations led by the Trump administration left out the elected leadership in Kabul, which “destroyed the Afghan government’s legitimacy” at a time when it already had little respect from local communities, said Watling."