For the same price you can get a Colt and while Colt isn't the end-all/be-all, everyone has heard of a Colt whereas not everyone has heard of a Sig. I have a friend who has been shooting for over 30 years, but it's mainly wing shooting. He mentioned a year or two ago that he might finally get an AR (these frenzies brought on by proposed legislation brings people out of the woodwork). I listed off some various brands to consider and his immediate response was "I'll get a Colt". He knows absolutely nothing about AR's but everyone and their third cousin has heard of the brand called Colt.
Those that know a thing or two about AR's already own one whereas people that don't know anything will see two rifles on the Walmart shelf: a Colt, a brand most Americans are familiar with, and 'the other one'. (disclaimer: I do not own any Colt products)
Right or wrong, If a Colt AR-15 is $1K, that's where all the other brands have to compete if they want that first-time buyer who is willing to spend more than PSA. If Walmart dropped the price on 6920's to $500, the Sig would instantly be $495.