Why would we move the best and 3rd best pieces for the 2nd and 4th best pieces?
General answer: Because a young top-30 defenseman is worth as much or potentially much more than a young top-10 but not top-5 forward due to asset-scarcity.
Specific answer: Calgary wouldn't. You're already solid on defense and thin up-front. (Which is to say you're in a similar situation to the Canes.) It makes no sense to move one of your best forwards at an area of weakness in a deal where the best asset in return is a defenseman at an area of strength.