According to members of the band, they tried to write singles after Syd left (Point Me At The Sky, anyone?), but it was a disaster, and they realized they weren't any good at it. So they didn't do it again. I don't know if that means they just weren't interested in that anymore; maybe they realized that the person who was good at writing singles wasn't in the band anymore, and that this particular endeavor was beyond their skill set.

Like Roger said, it's good they couldn't write singles, otherwise they wouldn't have done the interesting things that they did.