After 11 years in the note business, I have learned that the simple answer to this question is: better use of the cash.
For business people, they will usually examine their options when looking for cash to make another investment or take advantage of a business opportunity. If the price for the note meets their expectation, they will sell. Otherwise, they will choose another option.
For non business people, the process may be the same, but the reason for selling will usually be different. Maybe the need is a big medical bill, buy a new car, buy a new home, etc. Usually, more personal.
My job is to provide an option and for you to decide if it fills your needs.