http://hexus.net/tech/news/graphics/105892-amd-rx-vega-computex-event-confirmed-soft-launch/
 
So basically on the launch date at Computex 2017 for Vega the actual video cards will not be available that week. There are several possible reasons for this but AMD won't be pushed into making a premature launch if the card's firmware and/or drivers aren't ready yet. AMD also hinted about HBM2 memory availability which could also be a limiting factor as to the delay.