Who is "we?"
There's a lot of blame to go around for ignoring Afghanistan after the Soviets pulled out, most especially to be directed toward the United Nations and NATO.
Osama bin Laden, in particular, got pissed off about Saudi Arabia allowing US troops into that country during and after the 1991 Gulf War to liberate Kuwait, which led to him getting involved with Zawahiri, an Egyptian, who has been trying to overthrow the government of Egypt for a long time. Zawahiri is really the person who created what we now call Al Qaeda and recruited bin Laden to finance his operations.
However, the Saudis expelled bin Laden who subsequently went to Sudan. In 1993, the Sudanese government was finally sick and tired of bin Laden hangin out in their country and wanted to get rid of him. Knowing that he would flee to Afghanistan if kicked out of Sudan, the Sudanese government offered him up to the USA. Bill Clinton said, "No, let him go."
And that's the Reader's Digest version of how bin Laden ended up in Afghanistan. Of course, the real tale is far more complicated than that...and even Bill Clinton had legitmate reasons for not taking bin Laden when he had the chance.
Judging history with 20/20 hindsight can be enlightening, but not always fair to the players involved at the time.