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

[Taylor]

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 im honestly considering switching providers over this bs.