The Sub is recognized everywhere, probably, as Tom said, the James Bond connection has been a big driver, but Jacques Cousteau, Sea Hunt, Steve MQueen, etc., and great marketing had a hand in it also..
The GMT has always been in the shadow of the Sub and only chosen by travelers who enjoyed the convenience it offered for time zones..
It's only been since the introduction of the Ceramic GMT and it's upgraded bracelet that anybody really paid much attention to it.. I think that it's sudden rise in popularity surprised many, including Rolex..
Now that the Sub too is available with a ceramic bezel and modern bracelet, it's taken back over as Top-Dog and the GMT will need to take back seat once again..........