The Marathon is a rather large event due to it's international component. The runners cross the Ambassador bridge to canada, run along the windsor waterfront and return to detroit via the tunnel. If they only recently decided to book the hotel, it may have been difficult to reserve a block of hotel rooms with the marathon coming up in less than 6 months. Participants usually like to stay close to the start of the marathon which begins and ends 1.5 miles from the arena and most of the nice hotels are located directly between the two sites.
Can't believe there have been so many international events in this neck of the woods in a single calendar year!