Can someone explain to me why GW chooses to make a araby army for warmaster and not for any existing races in epic, say Necrons?

I do not begrudge the warmaster people their armies, but how can GW say that the commercial intrest in epic necrons is smaller than the one for warmaster arabians? Surley they must be able to make at least some metals (are the araby models metal by the way?) like warriors.

This is not a rant, I'm just curious.