Because the Knicks need players who fit with each other. And when you have someone as unique as Carmelo who is a "power 3" as opposed to a perimeter 3 and you have chandler, you badly need a stretch 4 or someone who can trade offensive roles with Carmelo so that Carmelo can play around the basket
The Knicks biggest problems are floor spacing, and it's because the pieces in the front court just don't fit together. 6-11 - 7 footers who can shoot and pass and be a threat that needs covered on the perimeter are hard to find- this trade would give them 3 of those types as well as having 2 of them young guys that still have 10 years left in the league.
This roster would make the Knicks much better now and down the road-
Motiejunas or daye or Villenueva
That's a roster with balance built around complimenting Carmelo and chandlers games- or just go forward with this video game/fantasy basketball idea of 2 star forwards that need the ball in the same places will just end up working out