Well if the extra pins aren't needed, it's wasted cost, especially if DB15 connectors were more expensive than DB9 at the time, and if they were making CGA versions (I have a Samsung CGA) at the same time it'd mean they could use the same plastic case on multiple models without any changes to the case (other than stickers).
A lot of my VGA monitors don't even have a plug, just goes straight in to the back of the unit. R,G,B,Hysnc,Vsync,Ground - that's only 6 pins out of 9.