I'm not being funny but whether or not an event is televised in the US has no bearing on how big it is worldwide. I bet at the time you could watch the French Open on TV in Europe (where there are a lot more people) but could you watch the Dallas event?
The Dallas event was televised worldwide. Read some of my other posts in this thread. Like the comments by Newcombe about his Dallas win. And the atrocious attendance at the French in the 70s.
I'm betting that Dallas was televised in more countries than Roland Garros in the 70s. That seems very likely considering the huge difference in prize money. Money dictates everything-fans, tv deals, revenue etc. When players are playing in empty stadiums for peanuts in one event & players are playing in packed 20,000 seat stadiums in another for much bigger purses, it stands to reason that tv networks worldwide would be paying more for the rights to televise that latter.
and BTW the French was 64 draw for a few years in the 70s(that's how tough times were for that event)
and the australian was between 56-64 draw until 1988. Kinda puts into perspective why so many consider them lesser slams.