The Arnold is in early spring when most bodybuilding fans are finishing their bulking phase and have maybe started their diets, so then at the expos everyone is still bloated from all the dbol and kfc from a long winter. Then the O is in early fall when the summer diets are done and the winter bulk up sessions begin so any definition you had mid summer is fading fast.
The only reason I can think of is that the venues are cheaper in the spring and fall then mid-season.
quick question When was the Olympia held in the 70's and 80's?