Because they didn't have the Internet back then.
That's the main thing. Also in some cases 30-40-50 years had passed between remakes. We're seeing remakes today of films and TV shows that are only 20 years old. And in the case of the planned big-screen reboot of BSG, not even a few years.
Back in the day the only people who were heard to complain were the critics, and believe me they bitched like mad when Scarface came out. And The Fly.
Also, we didn't have DVD and widespread cable networks making things like the original Scarface and The Fly ubiquitous. Anyone with $15 to blow can buy the original A Team seasons at Wal-Mart. And depending what part of the world you're in you can also access a lot of "originals" via things like Hulu.
Up until the mid-80s something like the original Scarface only showed up once in a blue moon on the late late late late late late show. Sometimes there might be a VHS kicking around, but it's not the same. Also in many cases the remakes were of very obscure films. If 5% of the audience who went to see Brooke Shields romp around in 1980s The Blue Lagoon were aware it was also a remake of a 1949 film which, in turn, was based on a novel, I'd be surprised. And if you found enough of those people who had seen the original, or read the book, to fill the corner booth of a Starbucks, I'd be stunned.
If you want to go further back, say to the 1960s, the 50s, and earlier where it was not uncommon for remakes to be made within the same calendar year, especially if it was a case of a foreign film being remade for American audiences, the situation was in many ways worse than it is today. But people back then didn't care. Movies cost maybe a buck to go see, and the novelty was still the kind. And there was absolutely no such thing as home video, so if, say, someone had made a Scarface remake in 1962 odds are maybe 10% of the audience would have even been aware of the original.
I'm sure when Hammer remade Frankenstein and Dracula most of its audience, while they may have been aware of the Karloff/Lugosi originals, likely had never seen them.
However today you look at someone doing, well, a remake of BSG. The percentage of the audience who would go to see such a film in the 2010s who were a) aware of and b) had seen either the original series or the Moore/Eick version, would be approaching 100%. And 99% of those people would be on the Internet either supporting or complaining about it.
We also have to make the distinction between a remake and a retelling. As far as I'm concerned retellings of classic stories such as Hounds of the Baskervilles, or Hamlet, are not remakes. Any more than the multiple stage productions of Shakespeare that happen every year. For me a remake is when a story made for the screen (big or small screen) is redone.
Which means Peter Jackson's version of Lord of the Rings is not a remake of Ralph Bakshi's Lord of the Rings, any more than his version of the Hobbit will be a remake of the Rankin & Bass cartoon.
Alex