They really hit their stride/been on fire since the end of 2006, and have stolen a lot of market share from other teen/mall retailers as consumers have traded down during the recession. They've had double-digit top line growth for 10 straight quarters.
Buckle carries many brands and covers a number of trends/styles - unlike Abercrombie, Aeropostale, American Eagle etc who only stock a single brand and cannot take many risks without losing consumer loyalty. Frequently Buckle merchandise is, in fact, more expensive.
For instance their t-shirts by Sinful, Affliction, Remetee et al are in the $50-75 price range. Their jeans by Big Star, Rock Revival, and MEK are all around the $120-135 price range. AEO, ARO and ANF cannot ask prices like that for their stale, un-evolving merchandise.