Developing a new memory standard takes a long time, work on DDR4 memory for example started in 2005, a couple of years before the commercialization of DDR3 memory, so it doesn't come as a big surprise that work on High Bandwidth Memory started a long time ago. KitGuru has written an interesting piece about how HBM came to be, and how this new industry standard was first conceived at ATI/AMD sometime around 2006-2007.