One, this was organized down to the last detail, similar to how things were in the days of the suicide bombers in the second intifada.
Another reason is that the organization didn't leave any mark for Israeli intelligence that would have allowed it to foil the attack before it even began. The bombing did fail thanks to the awareness of the citizens on the bus, but these suspects belong to a known and dangerous terror group, the Palestinian Islamic Jihad, who is known to pull off complex, professional detonations involving home made explosives, without having any security force realize what it's doing.
What's more frightening is that it happened right under the Palestinian Authority's nose, and one of the main suspects is in the PA's police program.
The PA's security mechanisms should have picked up on some of the intended sabotage that they were planning, even before they got to the stage of executing the plan. Shahada Taamri, the police cadet, was even jailed in Israel before, and the PA should have put an eye on him.
All of these issues raised many eyebrows, including the possibility that the suspects had outside connections, from the Gaza Strip or from other places. The Palestinian Islamic Jihad takes logistics inspiration and funding from Iran.
It's important to find out if this is the first sign of other types of attempts, or if this is just an exception, as well as finding out what was their motivation to try to pull off the attack specifically when it was done, during negotiations between the two sides.
The Shin Bet and other field operative groups that located the suspects, deserve full credit for their quick and professional work, using human and technological intelligence. The IDF performed its duties skillfully in arresting the men, but the whole affair is very disconcerting.