It’s quite rare that we see young players exchanged between two organizations. There is always the risk that the young prospect moved suddenly blossoms and makes the team look foolish for giving up too early. NHL teams are generally risk averse, so sending out pieces that are not completely known assets is probably perceived as even more hazardous.
At the time Montreal moved William Bitten for Gustav Olofsson, the defenceman had earned some time in the NHL but was still taking his first steps in the league. While closer to his ceiling than Bitten (the younger of the two), he also couldn’t be called a finished product.