Didn't the fact that a bloke on his own had four tiffin boxes raise any eyebrows?
Not sure how it happens in other countries, but in India its common for the man of the family to carry a bag full of tiffin boxes, water bottles, and snacks for his family to the convention. I myself carried my family bag which had 3 tiffins and 4 bottles and small packets of chips for my and my wife's family.
The bomber's in-laws were there in the audience and he pleaded his wife in the morning not to let them go to this convention. Hence, he had to strategically place the bombs away from his in-laws.
Also, rather than using shrapnel, he placed 2 liters of petrol near the bombs to inflict maximum damage. His detonator was amateur, so he had to be within 500 metres of each explosive to detonate them.