[ghaff]

>people travel to the destination from all over the world and book hotels in the city where they can easily get to the start location.

Or at the end. The Boston Marathon actually starts pretty much 26 miles to the west of downtown in a suburb/exurb. So runners do need transportation to get to the start. It's not really accessible by public transit but I imagine there are buses in addition to people just getting rides from friends/family.

It is held on a (state) holiday and a lot of Boston companies give the day off. (And companies out by the start like EMC do as well because their parking lots are used.)