There was no great reason to think the second tower should be evacuated, until the second plane hit, we all just innocently thought it was a horrible flight accident. Until the second plane hit, we were thinking it was an eery repeat of the accident in which a plane was flown into the Empire State building in the 30's. After the second plane hit, was when they started to look for others, and found out about the one for the Pentagon, and the one that crashed short of the White House.
It's all too easy to look back on it in hindsight, and say, it was obvious, when at the time, it wasn't thinkable that it could happen, here.
America lost a lot of innocence, that day.