Obviously the Olympic Gold is only available every 4 years. But you can be mediocre all year and just get hot for the World Championship or Olympic Games. Diamond League is the only one that requires consistent high level performance during a season long competition. Also I'd be interested in how much $$ is actually available to athletes for winning Oly or WC titles, because Diamond League wins bring along a very pretty payday.
Have there ever been runners that were widely accepted as "dominant" that haven't won DL titles?