Because although the Taliban are religious fundamentalists, they are real Afghans with deep roots in Afghan culture and history.
The US coalition lost because while they were welcomed as liberators in 2001, they stayed too long, and became perceived as occupiers. The only reason young Afghan men had for joining the Afghan army was because they had no jobs, and they would get food, room and board and a small salary which they could share with their family by joining the Afghan army.
However, they had no loyalty to the Afghan government because they just wanted to milk this cow as long as it could produce milk. When it stopped producing milk, they had no intention of fighting and dying for the Afghan government.
The US government spent billions in Afghanistan, but most of that money was likely siphoned off to private bank accounts, and did not make its way to ordinary Afghans. I am sure that there will be more than a few pro-American Afghans who will retire to very nice homes in Qatar with their families after the fall of Kabul.
The long war in Afghanistan was just a scam which made lots of US military contractors rich, and some Afghans rich too. That is what happens when US voters believe that making business people their political leaders is good because they believe that government should be run like a for-profit business. The Afghan war was a 20-year business project with a lot of contractors, and American voters are the suckers because they got stuck with the bill. It’s that simple.
This was exactly the same reason the US lost Cambodia and South Vietnam. I wish I could explain to you why the US media and most Americans have trouble understanding this simple fact, but I can’t.
The Taliban were not fighting for money, but for their country. You can like them or dislike them, but this is what happens when one side fights for profit, and the other side fights for their country.
Taliban = Islam Fundementalists
What's wrong with a Taliban Government for their citizens = Islam fundamentals
Root cause = Islam