not everyone wants to buy the device with the most new innovative features or technologies. Not everyone wants to buy the device that adheres to the most stringent open, cross-platform standards or paradigms. Some do, and for those the iPhone is not the right product.
Others want something that looks sleek, contains the necessary features for them (and has the option to add further features through an app store, which no other product did, it should be noted), and just works. For those, Apple's obsession with full control of hardware and software and somewhat whimsical approach to approval procedures to third-party apps might not matter that much.
Let's try to avoid turning this into a pro-/anti-Apple flamewar. The iPhone has its flaws but it's still a damn good product.
That Guy above is probably spot on, Apple may well have been influenced/persuaded through some gentle nudging by the various carriers that shelled out boatloads of cash to get exclusivity on the iPhone and see Google Voice as a direct threat to their revenues.