The price to acquire Juan Soto was too much to pay
I broke this down a bit last week, but in the days following the 2022 trade deadline, a narrative built up that Dylan Carlson was the reason a trade for Juan Soto did not work out. While Carlson was certainly a part of the negotiations, the package would have been far more than the young outfielder.
Names like Jordan Walker, Masyn Winn, Tink Hence, Nolan Gorman, and potentially more could have found themselves in that deal with Carlson to acquire Soto. The internet has a bad habit of armchair GMing major trades like this and saying "Just give them Carlson, DeJong, and whatever two or three prospects I don't believe in". That's not how trades work...unless you're the Colorado Rockies or Arizona Diamondbacks.
Seriously though, when you're talking about trades for superstars under team control, the packages it will take to acquire such a player will hurt (keep that in mind as you think about a potential move for an ace). Teams are not going to move on from elite talents for nothing, barring some weird front-office decisions.
In the case of Soto, the Cardinals would have had to basically wipe out most of their young talent for just his services. Yes, he's elite, but not worth that kind of package for St. Louis.