The iPhone is a tremendous cash cow for Apple, and they've learned that they can get by with doing very little upgrading to the basic model. They have no incentive to change as long as everyone keeps buying the same old design over and over again. So all of you clamoring for change, stop giving Apple your money!
I genuinely hope you're trolling, otherwise, that's just ignorant.
There are several widely accepted arguments regarding why Apple doesn't produce phones (and even tablets) in multiple sizes:
1.) Apple isn't Samsung; they don't want to have a different phone for every person's idiosyncrecies. They just want to make the best phones. It's widely reported that Ives and fellows make dozens of working models of most Apple tech and finally decide which one is best, and
that's the model that gets manufactured. For some people this is a weakness that Apple isn't the first to the party and doesn't have 4G, 3D, kickstand, bla, bla, bla, etc. I don't think Apple sees it that way at all.
2.) As has already been mentioned, there are also developers to consider with UI, pixels, ratios, and general standards of quality to be upheld. I think one can see with the upgrades to retina on iPhone, iPad, and now MBP, that these aspect ratios and pixel counts are a high priority.
Congrats if you read all of that and Happy 4th!