To give time for the runners to approach the finish line, and the crowd there to reach maximum size. I recall seeing that they used "egg timers" to set the explosives off. I'm not aware of any mechanical timers of that sort having very long maximum time settings.
A quick search shows that most such timers have a maximum time of one hour, so they would need to near the spot where they intended to set the bombs off up until one hour before the end of the race.
Yeah, but in this case the maximum crowd size was around 2:10 to 2:30 race time when the elite runners would be crossing the finish line. The bombs did not go off until the crowd was much thinner at 4:09.