Enter Wig. (Vader waits in the wings with a big hammer.)
Kordell Stewart was among the most gifted athletes to ever play football. Not that he was physically more talented than others specifically. There was a long history of players who had played iron man football or who had played a couple positions, but Stewart did something really unique. He played Slash. And more importantly, he did it well. He was a gifted runner, a talented receiver, and an adequate passer. When lined up in a formation that disguised him in such a way that he could do any one of the three things, it was extremely difficult for defenses to know what to do. Even when he was limited to either passing or running he was hard to contain. As a receiver he was still completely capable, and the Steelers used him in ways that allowed him to potentially catch screens and throw from behind the line.
Before he converted to a quarterback, Stewart was one of the most difficult players for defensive coordinators to plan for. He was to defensive coordinators what Troy Polomalu would later become for OCs. I once read an article about Stewart written at the height of his popularity just as he became the starting full-time quarterback of the Steelers in 97. It was unsurprisingly a fluff piece talking about all the things Steward did that caused headaches and had been compiled by talking to offensive and defensive coords around the league. Interestingly, the consensus among those coordinators was that Stewart was an exceptional athlete and he was good in any of the skill positions, but if he had a weakness in any of them, ironically it was at the Quarterback position.
Obviously we all know how it unfolded. Clearly Stewart wasn't able to read defenses well enough to excel at QB. Pete Carrol learned at the end of 97 during the playoffs how to beat Stewart. Basically you spy him with your most athletic linebacker and dare him to beat you throwing. (Carrol and his Patriots didn't beat the Steelers in that Wildcard game, but they played them to a 6-7 loss. and held the high-scoring and record-breaking Stewart to a season low in rushing and passing yards.) As it turns out, once Stewart begins to run, that's it. Unlike McNair or McNabb, he lacked the ability or forethought to make a throw after he began to scramble. So once Stewart broke from the pocket it was safe for the most part to abandon coverage and converge on him. That took away any advantage of his running ability and clearly his passing ability was never top shelf anyway. Between complicated defensive schemes that he couldn't quite read and the linebacker to keep him from scrambling, Stewart was a one or two read QB at best. If a receiver couldn't get open quickly, Stewart wasn't going to excel.
Now with a guy like Brown, who could run precise routes and get separation - Stewart may have been able to put up some good numbers, but he still would never have been able to be a top tier QB. Had Stewart remained "Slash" he likely would have gone down as one of the most loved Steelers of all time, rivaling Hines Ward. Ironically, his choice to convert to QB likely cost him fame and probably even some money over the long run. It may have even cost the team a championship or two.
At the end of the day, he simply wasn't meant to be a pure QB. He was supposed to be Slash.