I just don't think they are selling. The same six AR-15's have been in the display case at my local Walmart for six months.
When you can buy a lower receiver for $49 and an upper for $149...and customize the rest of the rifle any way you want...buying a $1200 Colt really doesn't look like that attractive of an option.
The Walmarts that still sell guns are in areas where there are a lot of gun owners and a pretty high number of veterans are available to help even the greenest of AR-15 neophytes build a great modular weapon at the lowest possible cost.
PLUS, most of us that already OWN AR-15s in 223/5.56 are looking for models that Walmart doesn't carry...like 308s or 300 AAC Blackouts.