A lot of big department stores and regional shops have awoken from their slumber and built right next to the local Big 5. Go check it out for yourself in your city. However, because Big 5 is only sporting goods, and the other stores are sporting goods and more, they will pawn Big 5 by cutting directly into their business while being fully diversified in other areas.
Basically, eat into the sporting goods business enough, and you kill Big 5. Other companies like Kohl's, Fred Meyer, etc. are already on top of this strategy.