Well if you are talking about why the growth has been as spectacular in the past few years then international growth is the reason. Apple only recently started to market internationally compared to the other tech giants (most hardware are located in Asia so they had that presence). There is so much untapped potential in China and some other markets its crazy to think about.
Certainly, but they've had their presence in the US and have been growing non-stop here too (even before they expanded to Verizon and Sprint). Growth has a lot to do with it, but that's not the only reason they're so successful. Why would any analyst be so baffled by their growth if the only reason was a greater global presence? Seems pretty obvious to me, and should be to them.