Not so sure about that...I remember albums were $3, less on sale. That was about an hour's pay for your average crappy job, just like the cost of a CD or album DL is about an hour's pay for the same crappy job today. More or less. Singles were something like 60 cents. So I don't really think that was it. The filler, yeah, maybe. But I think it was a holdover from 78s, which had been the standard for decades. Plus, radio was a strong factor, they played the hits, that's what the singles were.
David Kitching said:
The reason singles were so important in the sixties was that LP's were so expensive. Another reason was the already mentioed filler tracks, I'd hear people saying how they'd bought an LP and there was a load of boring crap on it. And they wouldn't buy anything else by them.