The cost of moving up in the draft can be expensive. A team manoeuvring its way into one of the top few slots is a rare feat in the NHL, and sometimes nets a less-valuable return than what was given up, often in the form of a lost roster player.
For every one of those trade-ups, there's a trade-down in which a team sends a comparatively high draft pick to another club convinced that the player they have their eye on will be gone by the time they get their chance at the podium. The return is often a package of assets, making up in quantity what the team above them is losing in quality.