Because you need things in the system, if you throw a dart at 15 balloons, there's a chance one hits. If you throw a dart at 3 balloons, odds aren't looking that good.
If you're not in the top two tiers of the draft, you trust your scouts and go with them.
If the Canucks were loaded with up and coming prospects, you don't need as many picks and are able to get rid of 2 or 3 to move up and take a guy you really like. If he doesn't pan out you've got 6 or 7 more coming.