Maybe they were concerned about the durability of the Matte Black finish compared to Space Grey? It may be fine on an iPhone, which is a small device and probably only used for 2-3 years. Imagine a 15" MBP being used for 4, 5 or even 6 years - especially considering they are usually used on desks which may have small pieces of grit etc. on and thus the potential for scratches.
Unsure why they didn't go gold, though. Maybe they didn't feel it was the right fit for an ostensibly 'Pro' machine, but having said that, the iPad Pro comes in gold and even rose gold. I might've considered gold just for a change if it were available, luckily I make sure I'm never in an environment where I'd be judged on the colour of my laptop...