Walmart has to contend with Best Buy's current promotion that allows Apple iPhone 4S and Apple iPhone 4 users with an upgrade in their pocket, to trade-in either of those phones and receive a free Apple iPhone 5. Best Buy tried that for one day and it was such a success that for 9 days starting yesterday (Friday) the promotion will continue to run.
Walmart and Best Buy could be looking to clear the decks before the next iteration of the Apple iPhone is introduced. Historically, purchases of the iPhone come to a halt between the introduction of a new model and its launch. The Apple iPhone 5S is rumored to keep the 4 inch screen, offer a faster A7 processor, better camera sensor, and possibly a fingerprint reader. Alleged leaks of iPhone 5S parts are beginning to turn up which usually occurs prior to the introduction of a new iPhone. There are also rumors that a low-cost Apple iPhone will be unveiled this year.
source: Mashable via MacRumors