For those that kept up with Afghanistan this quick victory came of no surprise. The Taliban effectively controlled all areas outside the cities by the end of the Trump administration. Only the cities remained effectively permanently besieged. Simple because they hosted the HQs of the Western forces.

IMHO the biggest problem of the West was always to reduce the Taliban to mere religious extremists. Religion is merely the glue that keeps the army together.

Afghanistan is a tribal society. 75% of it's inhabitants live in tribes in the rural areas. The Taliban understands itself as the "party" of the tribal leaders.
And for a tribal society, any modernization means a loss in power for the tribes. Whether it be women's rights, religious freedom, free elections. Anything like that reduces their political authority.

In that way the goal of the Taliban is to enact some sort of "Afghani Sakoku Edict": banning ALL foreign influence. Which includes women's rights and democracy. Islam is merely a tool for that. But because the West made it all about Islam, the political influence of the Taliban has been severely underestimated.

And finally: the Afghan government barely paid their army. If they did then often belated with less money than promised. The Taliban pays in time. The agreed sum. With bonuses if you got wounded and permanent pensions to your families in case you get killed. No shit tons of Afghan soldiers just switched sides.

And no, it's not Trump's fault, neither Obama's nor Biden's, maybe Bush's… the fate was most certainly sealed during the Cold War. Both the Soviet invasion and the US arming the mujahedin equally built into it.

Even though Afghanistan wasn't very developed it may have developed on it's own, just as the Islamic world is now slowly but steadily doing… but this invasion and subsequent "reconquest" effectively sealed it's fate for the next 50 years if not century: they won't develop anytime soon…