Attention!

The three-minute song exists because of the limits of attention, not necessarily the result of the constraints inherent in a medium. Songwriters are still making them because that is a sufficient amount of time to spin out musical material. A three-minute section in a classical work has much more information in terms of musical development. Perhaps the reason songs got longer in the 70s was because classical music was an influence, not so much the result of the constraints inherent in technology. When duration is not so much an issue (as it is now), it really is about attention.