Quartz published a very interesting opinion on why the new Instagram video service tops out at 15 seconds (compared to Vine’s 6 second videos), citing 15 seconds as the length of a short TV commercial. Facebook, which owns Instagram, has long sought new ways to monetize digital content. This could very well be a play into the ad space generally reserved for television.

Source: http://qz.com/96475/its-no-accident-facebook-made-instagrams-new-videos-exactly-as-long-as-a-television-commercial/