For three reasons:
1. $440 million is a lot of money, but it was spread over 15 years. The average annual value was below $30 million, low for a player of Soto's caliber in today's market. It was below the AAV of David Price!
2. The offer was HEAVILY backloaded. It would maintain something around his current $17 million level for the first few years, but backload $40+ million salaries into the final years. Soto and Boras wanted it less backloaded.
3. The most important reason, according to Boras -- the ownership instability. He might have been more willing to sign if he knew who would be signing the checks. Apparently, this was major concern.
I think the new owners, whoever they may be, should have been given the opportunity to re-sign him. I hope that when he comes up on free agency, the new owners will try to bring him back.
Nonetheless, the Nationals got a HAUL.