Aaron Greenberg mentioned on the MajorNelson podcast that Quantum Break is a Windows 10 Store exclusive and is not coming to Steam.
http://majornelson.com/podcast/mnr-563-quantum-break-news-and-more/ (via Dual Shockers, at about 1:16:15 in)
http://majornelson.com/podcast/mnr-563-quantum-break-news-and-more/ (via Dual Shockers, at about 1:16:15 in)