It's about money.
Because the quality of silnylon is all over the walk, and due to US environmental restrictions on silcoating, the most waterproof stuff comes from Asia (although the nylon is usually DuPont - where are their factories?-no idea). Check this out in Richard Nisley's water resistance tests on this and the MYOG forums.
A large minimum order of silnylon in '28 different flavors' from Asia would put any cottage manufacturer quickly out of business. And the larger mfgrs. who outsource to Asia don't choose to offer much choice to customers, either. They're about money also.
And as you can see from this thread, people are all over the walk also about color preferences, for a mutitude of both aesthetic and (supposed) practical reasons.
So a medium gray(grey) runs into the least resistance from customers, and therefore has to be the color choice, or at least one of a few choices, for smaller mfgrs. to stay in business.
Personally, I like light emerald greens; but choosing between building a much better quality shelter or getting my color choice as a MYOGer is a no-brainer. So I'll take anything that doesn't make the shelter into a dark and dreary cave or a bright and garish eyesore. A medium grey/gray seems to be what sells the most for the mfgrs. They ought to know.
But I'll never understand the appeal of the darker shades, be they in grey/gray, green or brown, for shelters. And probably there are many out there who will never understand the appeal of the light emerald to me. For more discussion on this perplexing issue, you can look at a MYOG thread begun by yours truly at: http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=73002