Ah ok, I didn’t see where we jumped back to 1991. My mistake.
In that case it’s pretty straightforward. Hasek was not Belfour’s backup for any meaningful amount of time that year, Jacques Cloutier was. Instead of riding the pine in Chicago, where Belfour played almost the entire schedule, Hasek was given the starter’s crease in Indianapolis where he put up the best numbers in the IHL by a large margin (Guy Hebert being a distant 2nd, which gives us something resembling an NHL benchmark). Almost all of Hasek’s NHL playing time came at the end of the regular season, on an injury call-up.
Why send Hasek to the IHL and keep Belfour in the NHL in the first place? Pretty easy to understand — the Blackhawks wanted to take it slow with him, as he would have to deal with the learning curve of North American hockey, smaller rinks, a new language, culture shock, and so forth. Meanwhile Belfour had played the prior year for Team Canada and was a very well known commodity. And of course, Belfour proceeded to put up an all-time great season while needing almost no backing up. There was never a question of Hasek replacing him. The plan from the very beginning was to put Belfour in the spotlight and take their time developing Hasek and Waite, while using veterans as the NHL-level backups. Notably, the veterans Cloutier and Greg Millen were both gone from Chicago by the summer as the Hawks realized they had too many NHL-quality goalies.
To underscore that last point, it was well understood at the time that Hasek was destined to be not just a future starter, but a very good one at that. He validated that belief at every step, as the clear-cut best goalie in Europe, then the clear-cut best goalie in the IHL, then being the only NHL goalie with more than one GP who posted better numbers than Belfour. Given the plan that the Hawks had laid out for him, he literally could not have done anything more than what he did to prove himself. Very similar story to Tim Thomas a couple of decades later.