the ATF was about to be absorbed into the FBI, so to prove that they were still needed as a separate bureaucracy the AFT decided to make a show of how good they were, they invited the media( thats how koresh found out about the raid), the AFT called the raid "operation showtime".
after the ATF blew it the FBI took over, they borrowed the armored vehicles from the national guard by claiming it was a drug bust. otherwise the FBI could not have used the guard, (posse comitatus).
the FBI said they had to act because it was going on too long and they had to end it.
years before, the indian stand off at wounded knee went on much longer and no more people died at wounded knee because the president at that time sent in a army general to take charge and his orders from the president was to settle it without loss of life.