Probably financial reasons and also, despite the clean sheets, he couldn't be 'trusted' to play the way Ten Hag does. The modernity of football means keepers have to be able to be good with their feet as well as stopping the goals. De Gea appeared to lack that. I rate De Gea but when he does make a blunder (which seemed to become frequent), it does affect the team. Maybe they thought now was the best time to cut ties and rake those wages back in. The thing I don't like about what Man Utd did is offering the contract to him, getting him to accept it, and then taking it away and pulling out of it. That's not professional and actually quite bad in my opinion.
Onana seems a better prospect for Ten Hag's style. Agree with you that whoever gets De Gea on a free is getting the better part of the deal.