I think the issue is the structure of these Marathon races. The appearance fees get sucked up by one big athlete and there isn't enough to get all the top guys in one race. Berlin breaks the bank for Kipchoge every year. Last year the field outside of him was not that strong, 2nd place was 2:05:58 and Ive barely heard of most of the guys in the top 10.
London usually has the deepest pockets to get quality fields and Valencia has stepped up in recent years with some big names too. But not sure there is enough $ to go around to get all these guys in the same field, which is a bummer for fans.