The US had publicly stated we would depart by September 11th, and from there the President moved the date to the left to August 31st. Starting in June the US and Nato allies began closing down our bases. At the very beginning of July Bagram closed, and all military bases in the country used by Nato had been turned over. Concurrent to this was the ANDSF losing district after district to the Taliban. Recognizing the country was in peril, assets went to HKIA to protect the embassy. Despite air strikes, the ANDSF was unable to stop the loss of ground. By early August several critical cities were surrounded and unable to be resupplied. Within a couple weeks, several fell to the taliban including Kunduz, Herat, Kandahar. Several of these fell without any fighting. The momentum became too much and the entire government of Afghanistan collapsed leading into mid August. US forces were not prepared for this, and evacuations had not been occurring publicly to prevent panic. The Taliban surrounded and took Kabul, and through negotiations, allowed the US to leave by the 31st. And from there it's been what you see on the news.

There's a lot of reasons why people are still there. Some had flights for dates after the taliban took over and those were canceled. Some, like most of the US, got caught flat footed when the ANDSF collapsed and had a long road to Kabul to try and escape. Even those in Kabul may not have had the means to get to the airport safely.