The simple explanation is that the bombers waited for the...

The simple explanation is that the bombers waited for the sniffer dogs and bulk of security to go home

That is why the bombs were planted after the race was won, the medals presented and the VIPs gone home - and security presence relaxed even though there were still the marathon stragglers and many spectators. When the sniffer dogs and security was returned they found the remaining bombs.

By the way it is looking like Al Queda in the Arabian Peninsula was responsible. The three Saudi suspects were flatmates (one still on the run - why Logan airport flight was delayed, and one suffered burns to his hands that day). They are reportedly from the same Tribe near the border with Yemen that is known to be hostile to the Saudi royal family

Rumot