Because Hackett costs a lot less and his salary doesn't count against the salary cap. The Broncos could get rid of Hackett. They are stuck with Wilson.
Late in the very first game of the season, Hackett was already getting blamed by the national media for things that were clearly not his fault. He sent in a fourth-down go-for-it play on the Broncos' last drive, and Wilson's teammates were clearly waiting for him to call the signal to start the play, some even looking back to see what the hell he was doing. But Wilson

chickened out. It wasn't an attempt to draw the Seahawks offside, because Wilson didn't even attempt the "hard count." When the Broncos called time out with the play clock expiring, Wilson went to the sideline and straight toward the bench. He made no attempt at all to talk to Hackett and argue for going for it, like you'd expect somebody who "lives for" such moments to do. And then, after Carroll called a time out to "ice" the kicker

(and the Broncos took a practice kick, which should have been a penalty), Wilson had even more time to discuss a possible go-for-it play with Hackett, but made no effort at all to do so. And then all the national media jumped on Hackett for having his kicker attempt a 64-yard field goal at sea level. Pay no attention to the
yellow Quarter-Billion-Dollar Man. It's Hackett's fault, they all declared, and what a terrible decision! Never mind the talk of Wilson going to Denver being not about the money, but about "control." Never mind that instead of a normal coach-player relationship, they said they had a "partnership." No, no, no. That only applied when they thought the Broncos were going to win a lot of games and make noise in the playoffs. When things went wrong starting in the first game, it was all Hackett's fault, and Saint Russell the Blameless escaped scot-free.
Edited to add: once the Broncos had played several games and the Wilson-led offense continued to suck, a lot of the national media caught on that without Carroll's protection (derisively called "Peteball" by Wilson fans), Wilson isn't a top-75%-of-the-league QB, much less a top-tier QB. But they didn't stop giving Hackett a sizeable portion of the blame for the utter suckitude of the Wilson-led offense.