Frankly questions such as this aren't usually answerable until decades have passed and the archives of the internal deliberations in the administration are made available for historians to study... and even then historians may disagree.
Now, if you want an educated guess: the administration probably believed its own story that the Ghani government won't fall quickly: the official line was even that fall was "not inevitable". Various numbers I've read is that they were expecting it to last six to 12 months or so... That would have given them more time to sort out bureaucratic stuff like this.
The Soviet-backed regime of Najibullah managed to outlast the Soviet troop withdrawal 3 years (it actually only fell after Moscow stopped supplying it with ammunition and fuel), so American "hubris" that their ally would manage at least 6 months was perhaps somewhat excusable...
FWTW, some quotes in this direction:
Another Pentagon official interviewed by AFP said that diplomats had tried to speed up the visa process — but the process was too long and complicated under the circumstances.
The Biden administration assumed that the US embassy in Kabul would remain open and that the Afghan government would retain control of the country for months after the US withdrawal, he said.
But by mid-June the administration still did not consider an evacuation necessary and favored the granting of special visas — a process that can take up to two years.
It was only at the end of June that the White House raised the possibility of evacuating the Afghan interpreters before the end of the military withdrawal, and asked for the Pentagon’s help.
A crisis cell was then set up to organize the reception of Afghan refugees on US bases as they waited for their visas to be issued.
State Department spokesman Ned Price said that, when the administration realized that the situation was “quickly evolving,” it launched what Operation Allied Refuge, which he described as “a gargantuan US effort not only to process, adjudicate and to grant visas to so-called Special Immigrants but to actually bring them to the United States with a massive airlift operation.”
He said 2,000 Afghans have been brought to the US through the airlift so far [August 17].