If they went based on historical information and data, hijacking a plane and crashing it into a building was not something one had really seen before. With limited resources and intelligence you focus on what you know has been the weapons of choice first, and set aside a small cadre of intel folks on the rest.
That does not excuse the lapse, but it does help to understand why a little better. All the fbi reports I have read (and saved) give only brief mention to the hijack idea. Most hijackings were more for money or to free some political detainee. Most domestic terrorism were car bomb types and other items (the biggest ones being vehicle bombs as mentioned). The potential for more of those was there, as well as some of those containing chemicals.
When Okie city was bombed, we saw it as a tragedy and the work of evil people. The fault was with them primarily. With 9/11 the left wants it to be Bush's fault. I doubt those on the left, particulary at du and such, wanted clinton out for the oklahoma city bombing. People do things you don't expect (even though 2 such incidents occured during clinton's tenure on American Soil).
The focus should be on what went wrong, root cause analysis, and how to better handle things in the future. Presidents come and go, our intelligence community sticks around and keeps working. Those are the people who failed in preventing this and they should, and have, analyzed why and how to fix it.
Another attack may occur, but the probablility of such has declined. Despite our best laws and intentions, we still have a high murder rate. The wackos are out there trying, only a matter of time before one suceeds again - but I am more confident of the vigilance we are seeing now than I was when Clinton was in office. And Bush at least does not take such things laying down, he hits back.