this simply shows job not well done. to prevent attacks is their intelligence job.
no excuses !!
Aisay keh rahay ho jaisay har lallu panju ka kaam hai.
You tell me yourself, how hard is it to go and get 1kg of explosives in Pakistan?
Intel just tells you that an attack is going to happen. It does not tell you the route the attacker will take or whatever...they give estimates, an intel report is no more than an educated guess.
Let me give you another example. A intel report came a few months back that a suicide bomber had entered Islamabad through Tarnol side. All the police and others came into action and started combing the areas. The intel report gave a very rough sketch of the appearance of the guy and his target. The bomber was arrested with no damage done.
Everyday tens of reports come, only a couple are genuinely a threat, others turn out false.
It isn't an easy job, hand grenade blasts, crackers, 1-3 kg blasts, all do occasionally go unnoticed.
But almost every big terrorist attack has been predicted and on alot of occasions thwarted. I don't think you appreciate that. Just a couple of weeks ago 125 kg of explosives was recovered from a truck. That is a success.
You also mentioned that why don't the agencies stop the attacker from the source? That is a good point...and the answer is pretty simple. A suicide bomber can never be stopped, or mostly never. You can only make the attacker change his point of explosion but in 99% of the cases you can't stop the guy from exploding.
If not at the VIP target, he will explode at the check post or some place.