[Huliwang]

did he forget to evacuate them or because the withdrawal was urgent?

The timing of the withdrawal operation  was done secretly and orders were only given at last minute to prevent leakage. So everything happened suddenly and those left behind were not even aware of the sudden withdrawal until the Taliban matched into Kabul.

We can consider those stuck in Kabul were led down by Biden administration and this was not the first time US done such thing......