I wonder if they agreed to trade him before July 1st.
With a player like him, the Rangers or other teams would not want to qualify him and leave arbitration open as a possibility. He is one of those players who is in the wheelhouse where they aren't going to get a number high enough that their team could choose to walk away, but they might get an inflated number based on how the criteria admissible in arbitration paints a more positive picture than their current value.
He is easier to trade with a cheap 1 year contract and cost certainty.
As far as why Kakko didn't refuse to sign and play chicken with the Rangers to see if they would leave him unqualified, odds are he wasn't getting 2.4M in free agency.