Because releasing it together with the promo on the radio playing, the song's going to get a lot more attention. For example, "Radioactive" was released a second time to radio in October 2012. Within that timespan, a music video for "Radioactive" and a shit-ton of performances by the band on the radio, television and whatnot of "Radioactive" made the song extremely popular, whereas the single itself, a CD single released only in Germanic countries, was released in May 2013. "Demons" had multiple radio releases in early/mid 2013 in January and April, therefore a music video and performing the song during this time would push the song into the public sphere. It would only make sense to release a single if it's commercially viable, and they did in October 2013 after the song gained massive popularity worldwide. This time, however, "On Top of the World" was already released as a single in March 2013 in Portugal, and already has become massive worldwide. A music video now is just the icing on the cake, really!
^And a year later they're still promoting Radioactive EVERYWHERE. But they're doing it well. 6x Platinum.