Searches are cyclical. Just type words like "cupid" or "chocolate" into Google Trends and watch the spikes spring up every year around February 14. "Earnings" yields quarterly peaks, and the word "bored" - which for some reason is most popular in Australia - jumps every year around Christmas. So I put in the word "marathon." No particular marathon - just the word. And while the New York Marathon certainly shows up every November (sometimes as a double-peak with Chicago), it's nothing compared to Boston in April - despite the fact that Boston has fewer participants because (1) you have to qualify to run and (2) it's not a particularly fast or easy course.
What to conclude? Boston attracts more popular interest. And marathons like Berlin and London aren't even registering in the public consciousness.