It was just the next logical step from making succinct pop songs. What do you do after that? You make pop songs that are longer and more epic, that push the envelope. Imagine your favourite song, or something that you play over and over in the car, except that you don't have to start it over as much.
There's so many FM hits that I love. Bob Seger, there's two of his songs that I love. I would probably love more, but I don't sit around listening to Bob Seger records. It's the same thing with Tom Petty; he writes amazing hits, but it's not often that I sit around at home listening to a whole Tom Petty album.