Why doesn't T-Mobile allow the Iphone 4s in my area?

I don't understand. Please explain.
It says this…
The iPhone 4 and 4S are not available in your area. Please select the iPhone 5 to fully experience T-Mobile's advanced nationwide 4G network

The tmobile iPhones are different from Verizon, AT&T and sprint and other carriers. They don't have the same network as they do only in certain areas. I know it sucks and its kind of stupid. But they might have it in your area soon, tmobile just needs more time to step up, at least they finally got iphones! I'm a tmobile carrier as well and I have a windows phone and I want the iPhone 5! But I have to wait till my contract is up: p

IPhone 4 is 3G, iPhone 5 is 4G. So if T-Mobile doesn't have 3G in your area, you can't use the 4.