Lee's explanation is:
"Two reasons: one, I wouldn't go through the pressure and publicity I went through with To Kill A Mockingbird for any amount of money. Second, I have said what I wanted to say and I will not say it again."
BTW, Lee published a few short stories after To Kill A Mockingbird.
If you know about writing and publishing, you know this statement is nonsense. J.D. Salinger continued to write and publish while living like a hermit.