Because it has an “ecosystem” where Apple and others continue to milk you for your funds?
Because every schmuck on the planet has an Apple approved iPhone accessory?
I mean the phone sold maybe 30 million units in all of its incarnations. Other phones have sold more.
So what metric… do you mean the mind numbing drumming and brain washing that’s been incessant since the release of the rumors of the phone?
The iPhone is not the dominant device on the market… it is the most dominantly marketed device!

Frankly, even if no other device outsold the iPhone in direct numbers, there’s no reason to dismiss the RIM or Nokia products because they use the same OS for various devices, and in the real world, people prefer choice of form factors. 5:1 it seems. Not even counting the WinMo / HTC Samsung LG phones which would likely make it 6:1 or 7:1.