Why keep passengers on plane during bomb threat probe?

It was reported that a Singapore Airlines (SIA) flight landed at Changi Airport at around noon on Nov 22 but that "passengers were not allowed to disembark until around 2pm" because of a bomb threat ("Bomb threat causes 2hr wait to leave SIA plane"; Nov 23).

I do not understand this procedure. If a bomb was said to be on the plane, shouldn't the passengers be allowed to disembark as soon as possible, following which security personnel could do a thorough search of the vacated plane?

What if there really was a bomb on the plane and it exploded during the two hours while the passengers were still on it?

Steve Goh Han Choon