I don't know if anyone else has noticed this, but an explanation for the limited amount of Space Gray 6+ availability may come from the availability of the 6. If you take a look at the availability per this web site:
http://www.seaturtle.org/idevice/iphone6_grid.shtml, Most of the iPhone 6 availability is in the Gold, followed by silver and then space Gray. If Apple used the same percentage of distribution for the 6+, this could explain why it's availability is so poor in comparison to the other colors. Another factor that probably makes it even worse is that the SG is the only model with a black front.