Probably the same reason "Insert Show HEre" never went outside "Insert Main location Here", Familiarity.
On a side note: if we ignore all of the first (meaning DiC) dub episodes that were not a part of the original 65, we theoretically could place the setting in the Philadelphia area (Just gotta ignore the Tokyo Tower in the background)
Anyways, My silly reply to an even sillier question, Over.
Edit: on yet another note, the anime was also being an adaptation of the manga, which took place almost entirely in Tokyo, with the bad guy's lairs being in the north pole, nemesis, Tokyo Bay, (an airship?), and somewhere far off.