I've got a feeling that the stadium loan is secured against ticket sales.
If we defaulted on the loan, Norwich Union could in theory repossess, and then sell the stadium on for as much as they could get. That wouldn't be for anything like the amount that the loan is worth. SLH would no longer be liable though. What would probably happen is that the club would reform as a new company (Portsmouth, for example, did this), and would then either have to come to some kind of arrangement to buy the stadium, or find somewhere else to play.
You can see, therefore, why NU would rather not have to repossess. Even if we had to reduce our payments for a while, that would be better for them than the alternative.
As to why those clubs didn't lose their grounds, the simple answer would probably be that they didn't have mortgages on them. Not sure what the situation was at Leicester.