It isn't just the sales numbers for GA and AA volumes, it is also the cost of producing them.  Just getting the source material can be pricey since Marvel almost certainly doesn't have any reliable film from most of those original issues.  And even in those cases where you could piece together a complete Atlas Age issue from Marvel's 1970s reprints, you wouldn't want to rely on it since those versions were often edited and colored differently from the original issues.  
  
With that in mind, the sales revenue for the GA and AA volumes have to be greater than for the Marvel Age volumes in order to turn a profit large enough to be worth Marvel's time.