I think it's actually pretty simple.
To get under, they needed to trade JD Martinez, Nathan Eovaldi, or Xander. They were close enough to the playoffs that they were reportedly asking for a ton for either Eovaldi, or Martinez. My guess is that the FO didn't value the offers they were getting enough to sacrifice a chance at the playoffs, so they didn't blow up the team. The value of getting under the CBT meant X to them. The value of a chance at the playoffs meant Y to them. The prospects offered had a value to them. At the time of the deadline (but damn it, probably very different just a couple days later) X + Z didn't = Y.
The fact that they were slumping into the deadline, and that they continued to suffer really bad luck almost immediately as the deadline passed, makes that equation look bad in hindsight... but at the time it probably made sense.