NIKE designed those uniforms in 1997 when they had the contract. Yes Reebok took over but the uniforms never changed.
And as long as we're talking about the Broncos and Nike, consider this: When the Nike folks designed that 1997 Denver uni set, fans quickly noticed that they'd embedded a not-so-subtle calling card into the pants: giant swooshes that became visible whenever a player extended his leg forward. So when Reebok began making the Broncos' uniforms in 2001, they were put in the bizarre position of manufacturing a uni that featured a built-in ad for their biggest corporate rival (a situation that has now continued for a solid decade). It'll be interesting to see what Nike does with the swoosh stripes when they reclaim the Denver uniforms for 2012. But it seems like a safe bet that they won't be returning the favor by studding the design with built-in Reebok vectors.