Why aren’t more people using Skype for the iPhone?

Skype has been available as a free App Store download for a little while now for both the iPhone and iPod Touch.  I don’t understand why more people aren’t using it as their exclusive method of calling, or at least using it in the same numbers as those using the desktop version of Skype. Usage of Skype on the iPhone as a percentage of iPhone users is much lower than that for the desktop version.  I have a theory that I’ll get to shortly.

For three bucks a month for the SkypeOut service, you can make Skype calls to any phone.  On the iPhone, it would essentially be a method of calling that doesn’t use minutes.  Granted, the reliability of the service is in question since it relies on a steady data connection, but for non-important personal calls where all you want to do is chat, why not just exclusively use the Skype application?

I own a Verizon Windows Mobile phone as well as an iPod Touch.  I have tried using Skype on my Windows Mobile phone and it is disabled for calling (SMS works fine, though).  The iPhone on the other hand allows you to place calls from any Wi-Fi Hotspot, so why isn’t that getting more use, after all, its the same situation as you have when your physically at your computer (most computers aren’t connecting to the web through 3G, but through a physical connection)?

How come cellular services don’t switch exclusively to data-based voice communication?  Does it have to do with a lack of reliability?  Is it the need to drain every penny they possibly can from the customer?  Is it a combination of both?

For the record, I would use Skype exclusively if it were available for calls on any cell phone network.  As Comcast and others begin to roll out their WiMax networks (as they have this week in one market, and plan to do so in other major cities by years end), maybe we will have the ability to buy an iPhone thats not so connected to just one cell provider, and instead use it to dial through a standard net connection!  Then, cell phone companies would have no choice but implement a set monthly fee with none of these "per minute" charges.

Do you use Skype for the iPhone?  What is your experience with it?