Why through t-mobile am i able to get the iphone 5 in my area but not the 4S?

I go to the website and every time i type in my zip for the 4s it says its not available in my area. But the 5 just goes straight through to the purchasing process. Are they trying to force people to buy the 5 or is there solid reasoning behind it? Because i'm honestly considering switching providers over this bs.

There always a store.
And t-mobile got iphone when 5 was hot on market. They'll have like no 4s which is probably why you can't find one in your area.