You've asked the same question before, a few times in a few different ways. It's all laid out in the Initial Business Case Metrolinx published.
The subway will provide transit for the substantial number of riders north of Finch and even hugely more substantial new growth around Highway 7 {at least 50k new residents and 1000s of jobs). On balance, Cummer and Royal Orchard offered relatively few new riders and in the case of Cummer few new riders who weren't already within walking distance of either Steeles or Finch. Particularly with Royal Orchard, I'm not sure how adding it makes life easier for people living/working in Yonge to any significant degree. There's not much development potential, existing ridership or transit connections there and it's not like those people are forbidden from using the subway: they just have to get on one stop further north or south. In short, for the money it would cost to build the stations (and operate them, and the extra time for trains to stop at them etc.) you wouldn't be getting good bang for your buck.
Anyway, many here think there's a chance one of those stations still gets added (and Cummer would be my guess) but the extension makes plenty of sense as it is and probably would have made plenty of sense even when it was 3 stations. If you're asking sincerely, that's how and why the cost-benefit analysis says it's worth it.