Not that it really matters, but didn't everyone and their grandmother go see the original Batman? It was one of the biggest spectacles of our age, concerning film. Batman has always been a bit more popular than Superman on top of that.
So, a lot of people were really pumped for the first major motion picture Batman flick. It featured his greatest enemy, both played by some of the most well known actors of the age, although Keaton may have been more of an up and coming.
Plus, comics were a relatively big thing around that time. So, after word of mouth and all the trailers and promotions, it was like flies to ****. These days people have already seen TDK and Batman Begins. It's not surprising that TDKR hasn't sold "quite" as many tickets as Batman '89, despite our species ****in' like jack rabbits, lol.