Because that would have left him even further out of pocket. As it was he paid off £22m or so of the SBC debt from his own resources (on top of all his other input, a further £23m was paid by cashing in a bond Sunderland had had to lodge with FPP. That left the £25m. That got moved because he was intent on presenting the club as debt free. It wasn't, other than cosmetically. The first draft of the contract (as revealed by CM at a supporters' groups meeting) even had Short being paid that part of the parachute direct, until it was realised that PL rules wouldn't allow it. Short did it because he only ended up a few million extra down (the share price plus the £25m, less the £22m he did pay off) on the amount of internal debt he wrote off. It left him £25m better off. Could other offers have met the £40m price, but were unattractive to him in other way? You have to think either they didn't, or there were other aspects to those bids which he found unacceptable/
I’m pretty sure that it must have suited his overall tax planning at that point of time. I just think that he had concluded that the time was right, he could plan and exit but he couldn’t generate interest from the financial big boys whilst Bain had effectively ran the club into the ground. This was probably the only scheme that he could make work and it would actually secure revenue into the future whilst clearing the debts / tax right off, all at the same time.