As an NHL GM, your job is to put together the best team possible in order to compete for a championship. It's a complicated job with a very simple goal, win. While building through the draft is important, it's also imperative to supplement your team with trades.
Look at a team like the Pittsburgh Penguins, they are in the mix every single year and they almost always improve at the trade deadline. That's not a coincidence.
With that in mind, imagine closing off interactions with a team based solely on their geographical location. For the Islanders, you can't trade with the Rangers.